text: user1586: Reactome uses a three letter code for each species which are listed below.
  Are the letter codes used by Reactome a standard?
  If so is there a web page or download file that can validate the code and translate it to a common name?
   HSA  Homo sapiens                          
   SSC  Sus scrofa                            
   GGA  Gallus gallus                         
   DME  Drosophila melanogaster               
   RNO  Rattus norvegicus                     
   DDI  Dictyostelium discoideum              
   MMU  Mus musculus                          
   ATH  Arabidopsis thaliana                  
   TGU  Taeniopygia guttata                   
   DRE  Danio rerio                           
   OSA  Oryza sativa                          
   BTA  Bos taurus                            
   XTR  Xenopus tropicalis                    
   CFA  Canis familiaris                      
   SCE  Saccharomyces cerevisiae              
   PFA  Plasmodium falciparum                 
   CEL  Caenorhabditis elegans                
   SPO  Schizosaccharomyces pombe             
   ROT  Rotavirus                             
   FLU  Influenza A virus                     
   HCV  Hepatitis C Virus                     
   HBV  Hepatitis B virus                     
   HER  Human herpesvirus 1                   
   NME  Neisseria meningitidis serogroup B    
   NME  Neisseria meningitidis                
   CTR  Chlamydia trachomatis                 
   STY  Salmonella typhimurium                
   STY  Salmonella enterica                   
   ECO  Escherichia coli                      
   OCU  Oryctolagus cuniculus                 
   XLA  Xenopus laevis                        
   CAE  Cercopithecus aethiops                
   CGR  Cricetulus griseus                    
   HER  Human herpesvirus 8                   
   MCV  Molluscum contagiosum virus subtype 1 
   MCV  Molluscum contagiosum virus           
   HCY  Human cytomegalovirus                 
   HIV  Human immunodeficiency virus 1        
   PCH  Penicillium chrysogenum               
   TAE  Triticum aestivum                     
   VRA  Vigna radiata var. radiata            
   VRA  Vigna radiata                         
   CFS  Crithidia fasciculata                 
   FCA  Felis catus                           
   AMA  Arenicola marina                      
   MGA  Meleagris gallopavo                   
   NGO  Neisseria gonorrhoeae                 
   SAU  Staphylococcus aureus                 
   MVI  Measles virus                         
   HAL  Human alphaherpesvirus 2          
  
  
  EDIT
  Not a complete answer as this only covers a subset of all organisms but at present better than nothing. 
  Using KEGG Organisms
  https://www.genome.jp/kegg-bin/get_htext
  Enter letter code, e.g. GGA
  
  
  EDIT - After reading answer by conchoecia 
  The graph database schema for Reactome only has two labels that use the 3 letter code which is named abbreviation in the schema:
  Species and ReferenceTherapeutic
  Species has taxid  (Taxon Identifier)
  abbreviation     taxId       displayName                            
  AMA              6344        Arenicola marina                        
  ATH              3702        Arabidopsis thaliana                    
  BAN              1392        Bacillus anthracis                      
  BTA              9913        Bos taurus                              
  CAE              9534        Cercopithecus aethiops                  
  CAL              5476        Candida albicans                        
  CBO              1491        Clostridium botulinum                   
  CEL              6239        Caenorhabditis elegans                  
  CFA              9615        Canis familiaris                        
  CFS              5656        Crithidia fasciculata                   
  CGR              10029       Cricetulus griseus                      
  CPH              10703       Corynephage beta                        
  CPO              10141       Cavia porcellus                         
  CSB              60711       Chlorocebus sabaeus                     
  CTE              1513        Clostridium tetani                      
  CTR              813         Chlamydia trachomatis                   
  CVI              10243       Cowpox virus                            
  DDI              44689       Dictyostelium discoideum                
  DME              7227        Drosophila melanogaster                 
  DRE              7955        Danio rerio                             
  ECO              562         Escherichia coli                        
  FCA              9685        Felis catus                             
  FLU              11320       Influenza A virus                       
  GGA              9031        Gallus gallus                           
  HAL              10310       Human alphaherpesvirus 2                
  HAM              6706        Homarus americanus                      
  HBV              10407       Hepatitis B virus                       
  HCV              11103       Hepatitis C Virus                       
  HCV              31649       Hepatitis C virus genotype 2a           
  HCV              31646       Hepatitis C virus subtype 1a            
  HCY              10359       Human cytomegalovirus                   
  HER              10298       Human herpesvirus 1                     
  HER              37296       Human herpesvirus 8                     
  HGA              10376       Human gammaherpesvirus 4                
  HIV              11676       Human immunodeficiency virus 1          
  HPV              333760      Human papillomavirus type 16            
  HPV              337041      Alphapapillomavirus 9                   
  HSA              9606        Homo sapiens                            
  IBR              11120       Infectious bronchitis virus             
  LMO              1639        Listeria monocytogenes                  
  LMO              1906951     Listeria monocytogenes serovar 1/2a     
  LPN              446         Legionella pneumophila                  
  MCV              10280       Molluscum contagiosum virus subtype 1   
  MCV              10279       Molluscum contagiosum virus             
  MGA              9013        Meleagris gallopavo                     
  MML              9544        Macaca mulatta                          
  MMU              10090       Mus musculus                            
  MTU              1773        Mycobacterium tuberculosis              
  MTU              83332       Mycobacterium tuberculosis H37Rv        
  MVI              11234       Measles virus                           
  NGO              485         Neisseria gonorrhoeae                   
  NME              491         Neisseria meningitidis serogroup B      
  NME              487         Neisseria meningitidis                  
  OAR              9940        Ovis aries                              
  OCU              9986        Oryctolagus cuniculus                   
  OSA              4530        Oryza sativa                            
  PCH              5076        Penicillium chrysogenum                 
  PFA              5833        Plasmodium falciparum                   
  RNO              10116       Rattus norvegicus                       
  ROT              10912       Rotavirus                               
  SAU              1280        Staphylococcus aureus                   
  SCE              4932        Saccharomyces cerevisiae                
  SEV              11191       Sendai virus                            
  SPO              4896        Schizosaccharomyces pombe               
  SSC              9823        Sus scrofa                              
  STY              90371       Salmonella typhimurium                  
  STY              28901       Salmonella enterica                     
  TAE              4565        Triticum aestivum                       
  TGO              5811        Toxoplasma gondii                       
  TGU              59729       Taeniopygia guttata                     
  VRA              3916        Vigna radiata var. radiata              
  VRA              157791      Vigna radiata                           
  VVI              10245       Vaccinia virus                          
  XLA              8355        Xenopus laevis                          
  XTR              8364        Xenopus tropicalis 
  
  While ReferenceTherapeutic does not.
  The species can be identified using the NCBI taxon identifier, e.g.
  https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=6344
  
  
  EDIT - After reading answer by user172818
  Searching the file https://www.uniprot.org/docs/speclist.txt for 6344
  AREMA E    6344: N=Arenicola marina  
                   C=Lugworm  
                   S=Lumbricus marinus 
  
  or using the URL: https://www.uniprot.org/taxonomy/6344
  
  and with reference to
  
  I think databases should stop inventing new species abbreviations.
  
  The Wikipedia article on Lugworm shows the many Taxon identifiers in multiple databases.
  
  
  user37: These are not. The closest to "standard" is the 5-character abbreviation by Swissprot. It names 25,886 species and has been used for decades. It is also easy to remember for some common species such as HUMAN, RAT, MOUSE and HORSE. I think databases should stop inventing new species abbreviations.
  
  user2085: No, there are no standardized three-letter abbreviations for species names. Three-letter abbreviations with roman letters is 26^3 combinations, which is only 17,576 possible combinations. As a result there are not enough three-letter abbreviations for all living species.
  All three-letter abbreviations are either specific to a database or perhaps are shared between a database or two. There is currently no substitute for a binomial name (although NCBI taxid is a convenient parsable substitute for the species included in NCBI to date).
  
original_text: 1586: <p><a href="https://reactome.org/" rel="noreferrer">Reactome</a> uses a three letter code for each species which are listed below.</p>
  
  <p>Are the letter codes used by Reactome a standard?</p>
  
  <p>If so is there a web page or download file that can validate the code and translate it to a common name?</p>
  
  <pre><code> HSA  Homo sapiens                          
   SSC  Sus scrofa                            
   GGA  Gallus gallus                         
   DME  Drosophila melanogaster               
   RNO  Rattus norvegicus                     
   DDI  Dictyostelium discoideum              
   MMU  Mus musculus                          
   ATH  Arabidopsis thaliana                  
   TGU  Taeniopygia guttata                   
   DRE  Danio rerio                           
   OSA  Oryza sativa                          
   BTA  Bos taurus                            
   XTR  Xenopus tropicalis                    
   CFA  Canis familiaris                      
   SCE  Saccharomyces cerevisiae              
   PFA  Plasmodium falciparum                 
   CEL  Caenorhabditis elegans                
   SPO  Schizosaccharomyces pombe             
   ROT  Rotavirus                             
   FLU  Influenza A virus                     
   HCV  Hepatitis C Virus                     
   HBV  Hepatitis B virus                     
   HER  Human herpesvirus 1                   
   NME  Neisseria meningitidis serogroup B    
   NME  Neisseria meningitidis                
   CTR  Chlamydia trachomatis                 
   STY  Salmonella typhimurium                
   STY  Salmonella enterica                   
   ECO  Escherichia coli                      
   OCU  Oryctolagus cuniculus                 
   XLA  Xenopus laevis                        
   CAE  Cercopithecus aethiops                
   CGR  Cricetulus griseus                    
   HER  Human herpesvirus 8                   
   MCV  Molluscum contagiosum virus subtype 1 
   MCV  Molluscum contagiosum virus           
   HCY  Human cytomegalovirus                 
   HIV  Human immunodeficiency virus 1        
   PCH  Penicillium chrysogenum               
   TAE  Triticum aestivum                     
   VRA  Vigna radiata var. radiata            
   VRA  Vigna radiata                         
   CFS  Crithidia fasciculata                 
   FCA  Felis catus                           
   AMA  Arenicola marina                      
   MGA  Meleagris gallopavo                   
   NGO  Neisseria gonorrhoeae                 
   SAU  Staphylococcus aureus                 
   MVI  Measles virus                         
   HAL  Human alphaherpesvirus 2          
  </code></pre>
  
  <hr>
  
  <p>EDIT</p>
  
  <p>Not a complete answer as this only covers a subset of all organisms but at present better than nothing. </p>
  
  <p>Using KEGG Organisms</p>
  
  <p><a href="https://www.genome.jp/kegg-bin/get_htext" rel="noreferrer">https://www.genome.jp/kegg-bin/get_htext</a></p>
  
  <p>Enter letter code, e.g. GGA</p>
  
  <p><a href="https://i.stack.imgur.com/AOpHS.png" rel="noreferrer"><img src="https://i.stack.imgur.com/AOpHS.png" alt="enter image description here"></a></p>
  
  <hr>
  
  <p>EDIT - After reading <a href="https://bioinformatics.stackexchange.com/a/4939/1586">answer</a> by <a href="https://bioinformatics.stackexchange.com/users/2085/conchoecia">conchoecia</a> </p>
  
  <p>The graph database schema for Reactome only has two labels that use the 3 letter code which is named abbreviation in the schema:</p>
  
  <p><a href="https://reactome.org/content/schema/Species" rel="noreferrer">Species</a> and <a href="https://reactome.org/content/schema/ReferenceTherapeutic" rel="noreferrer">ReferenceTherapeutic</a></p>
  
  <p>Species has taxid  (Taxon Identifier)</p>
  
  <pre><code>abbreviation     taxId       displayName                            
  AMA              6344        Arenicola marina                        
  ATH              3702        Arabidopsis thaliana                    
  BAN              1392        Bacillus anthracis                      
  BTA              9913        Bos taurus                              
  CAE              9534        Cercopithecus aethiops                  
  CAL              5476        Candida albicans                        
  CBO              1491        Clostridium botulinum                   
  CEL              6239        Caenorhabditis elegans                  
  CFA              9615        Canis familiaris                        
  CFS              5656        Crithidia fasciculata                   
  CGR              10029       Cricetulus griseus                      
  CPH              10703       Corynephage beta                        
  CPO              10141       Cavia porcellus                         
  CSB              60711       Chlorocebus sabaeus                     
  CTE              1513        Clostridium tetani                      
  CTR              813         Chlamydia trachomatis                   
  CVI              10243       Cowpox virus                            
  DDI              44689       Dictyostelium discoideum                
  DME              7227        Drosophila melanogaster                 
  DRE              7955        Danio rerio                             
  ECO              562         Escherichia coli                        
  FCA              9685        Felis catus                             
  FLU              11320       Influenza A virus                       
  GGA              9031        Gallus gallus                           
  HAL              10310       Human alphaherpesvirus 2                
  HAM              6706        Homarus americanus                      
  HBV              10407       Hepatitis B virus                       
  HCV              11103       Hepatitis C Virus                       
  HCV              31649       Hepatitis C virus genotype 2a           
  HCV              31646       Hepatitis C virus subtype 1a            
  HCY              10359       Human cytomegalovirus                   
  HER              10298       Human herpesvirus 1                     
  HER              37296       Human herpesvirus 8                     
  HGA              10376       Human gammaherpesvirus 4                
  HIV              11676       Human immunodeficiency virus 1          
  HPV              333760      Human papillomavirus type 16            
  HPV              337041      Alphapapillomavirus 9                   
  HSA              9606        Homo sapiens                            
  IBR              11120       Infectious bronchitis virus             
  LMO              1639        Listeria monocytogenes                  
  LMO              1906951     Listeria monocytogenes serovar 1/2a     
  LPN              446         Legionella pneumophila                  
  MCV              10280       Molluscum contagiosum virus subtype 1   
  MCV              10279       Molluscum contagiosum virus             
  MGA              9013        Meleagris gallopavo                     
  MML              9544        Macaca mulatta                          
  MMU              10090       Mus musculus                            
  MTU              1773        Mycobacterium tuberculosis              
  MTU              83332       Mycobacterium tuberculosis H37Rv        
  MVI              11234       Measles virus                           
  NGO              485         Neisseria gonorrhoeae                   
  NME              491         Neisseria meningitidis serogroup B      
  NME              487         Neisseria meningitidis                  
  OAR              9940        Ovis aries                              
  OCU              9986        Oryctolagus cuniculus                   
  OSA              4530        Oryza sativa                            
  PCH              5076        Penicillium chrysogenum                 
  PFA              5833        Plasmodium falciparum                   
  RNO              10116       Rattus norvegicus                       
  ROT              10912       Rotavirus                               
  SAU              1280        Staphylococcus aureus                   
  SCE              4932        Saccharomyces cerevisiae                
  SEV              11191       Sendai virus                            
  SPO              4896        Schizosaccharomyces pombe               
  SSC              9823        Sus scrofa                              
  STY              90371       Salmonella typhimurium                  
  STY              28901       Salmonella enterica                     
  TAE              4565        Triticum aestivum                       
  TGO              5811        Toxoplasma gondii                       
  TGU              59729       Taeniopygia guttata                     
  VRA              3916        Vigna radiata var. radiata              
  VRA              157791      Vigna radiata                           
  VVI              10245       Vaccinia virus                          
  XLA              8355        Xenopus laevis                          
  XTR              8364        Xenopus tropicalis 
  </code></pre>
  
  <p>While ReferenceTherapeutic does not.</p>
  
  <p>The species can be identified using the NCBI taxon identifier, e.g.</p>
  
  <p><a href="https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=6344" rel="noreferrer">https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=6344</a></p>
  
  <p><a href="https://i.stack.imgur.com/oVwcY.png" rel="noreferrer"><img src="https://i.stack.imgur.com/oVwcY.png" alt="enter image description here"></a></p>
  
  <hr>
  
  <p>EDIT - After reading <a href="https://bioinformatics.stackexchange.com/a/4940/1586">answer</a> by <a href="https://bioinformatics.stackexchange.com/users/37/user172818">user172818</a></p>
  
  <p>Searching the file <a href="https://www.uniprot.org/docs/speclist.txt" rel="noreferrer">https://www.uniprot.org/docs/speclist.txt</a> for 6344</p>
  
  <pre><code>AREMA E    6344: N=Arenicola marina  
                   C=Lugworm  
                   S=Lumbricus marinus 
  </code></pre>
  
  <p>or using the URL: <a href="https://www.uniprot.org/taxonomy/6344" rel="noreferrer">https://www.uniprot.org/taxonomy/6344</a></p>
  
  <p><a href="https://i.stack.imgur.com/WpFQg.png" rel="noreferrer"><img src="https://i.stack.imgur.com/WpFQg.png" alt="enter image description here"></a></p>
  
  <p>and with reference to</p>
  
  <blockquote>
    <p>I think databases should stop inventing new species abbreviations.</p>
  </blockquote>
  
  <p>The Wikipedia article on <a href="https://en.wikipedia.org/wiki/Lugworm" rel="noreferrer">Lugworm</a> shows the many <a href="https://en.wikipedia.org/wiki/Help:Taxon_identifiers" rel="noreferrer">Taxon identifiers</a> in multiple databases.</p>
  
  <p><a href="https://i.stack.imgur.com/VWqBa.png" rel="noreferrer"><img src="https://i.stack.imgur.com/VWqBa.png" alt="enter image description here"></a></p>
   37: <p>These are not. The closest to "standard" is the 5-character <a href="https://www.uniprot.org/docs/speclist.txt" rel="noreferrer">abbreviation by Swissprot</a>. It names 25,886 species and has been used for decades. It is also easy to remember for some common species such as HUMAN, RAT, MOUSE and HORSE. I think databases should stop inventing new species abbreviations.</p>
   2085: <p>No, there are no standardized three-letter abbreviations for species names. Three-letter abbreviations with roman letters is 26^3 combinations, which is only 17,576 possible combinations. As a result there are not enough three-letter abbreviations for all living species.</p>
  
  <p>All three-letter abbreviations are either specific to a database or perhaps are shared between a database or two. There is currently no substitute for a binomial name (although NCBI taxid is a convenient parsable substitute for the species included in NCBI to date).</p>
  