Officer
An officer was a member of a group typically in the areas of the military. An officer's typical role was to command and direct, maintain discipline, and make plans and provisions for operation. Certain organizations had special criteria in order for one to achieve the status of officer.

Commission
In a military service, a commission was an order from the service's supreme authority, such as its commander-in-chief, authorizing an individual to legally hold a military rank, with all the privileges and responsibilities of said rank. A commission usually appointed an individual to hold an officer's rank. ()

In times of conflict, a temporary commission could appoint an officer to command rank to replace an incapacitated or killed commanding officer. (; ) Commander William T. Riker, for example, was granted a field commission promoting him to captain and given command of the  when the ship's commanding officer, Captain Jean-Luc Picard, was abducted and assimilated by the Borg during their first attempt to assimilate Earth. () Prior to this, Riker had repeatedly turned down commissions to captain various vessels in order to remain aboard the Enterprise and was considering turning down captaincy of the . ()

In situations where contact with Starfleet was not possible for very extended periods, a commanding officer had the authority to grant commissions to officers under their command, or revoke existing commissions; that is, they had the authority to promote or demote their officers. () Examples of this are when Captain Kathryn Janeway, commanding officer of the , promoted her chief tactical officer and security chief, Lieutenant Tuvok, to the higher rank of lieutenant commander, or when she demoted her flight controller, Lieutenant junior grade Tom Paris, to ensign. () After a year of exemplary service, she reinstated him to his former rank. ()

Within the Takret Militia resigning a commission was not tolerated. In 2152, the three Takret Rellus Tagrim, Renth, and Guri requested shelter aboard  to prevent their superiors finding them. ()

Starfleet

In the Federation Starfleet, an officer or commissioned officer was an individual who had attended and graduated from the four-year cadet program of Starfleet Academy. (; ) Upon graduation from the Academy, the cadet was granted a commission appointing him or her to the rank of Ensign, or in exceptional circumstances Lieutenant junior grade. (; ; ) Trades such as Doctor or Counselor graduated as a Lieutenant junior grade as standard due to the extra time spent studying before graduating. ()

Commissioned officers and enlisted officers were distinguished from other crewmembers through the use of a rank structure. (; )

Commissioned officer rank structure
Starfleet ranks from lowest to highest:
Ensign
Lieutenant junior grade
Lieutenant
Lieutenant commander
Commander
Captain
Fleet Captain
Commodore (aka rear admiral lower half)
Rear admiral (aka rear admiral upper half)
Vice admiral
Admiral
Fleet admiral

Other officer terms
Certain officer positions could be filled by non-commissioned officers and civilians if the need arose.

By rank
Field officer
Flag officer
Ranking officer
Provisional officer
Non-commissioned officer

By title
Senior officer (see also: senior staff)
Junior officer (see also: Staff officer)
First officer
Second officer
Third officer

By command structure
Commanding officer
Executive officer
Duty officer

By department or function

 Archaeology and anthropology officer
 Armory officer
 Bridge officer
 Communications officer
 Engineering officer
 Helm officer
 Intelligence officer
 Deputy intelligence officer
 Liaison officer
 Convoy liaison officer
 Fleet liaison officer 
 Medical officer
 Mess officer
 Morale officer
 Operations officer
 Personnel officer
 Police officer
 Records officer
 Science officer
 Deputy science officer
 Security officer
 Strategic operations officer
 Tactical officer
 Weapons officer

External link 
 

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de:Offizier
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ja:士官
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sr:Официр
 Officer
Occupations