Amnesia
Amnesia is a medical condition in which the patients lose some or all of their memories of past events. Physically amnesia is explained by the deletion or damage of memory engrams.

Humanoids
Hodin told Odona she would experience amnesia shortly before dying of Vegan choriomeningitis, saying, "You will see us, but you will not know who we are". ()

Captain James T. Kirk suffered from amnesia after being accidentally hit by a memory beam on the idyllic planet Amerind. His inner thoughts pictured his condition as follows:

"Where am I? What place is this? What are these? I feel I should know. They're familiar, and yet unfamiliar. How did I get here? Who am I? Try to remember."

"These last few weeks, my love for Miramanee grows stronger with each passing day. However, the dreams return every night. Fragments of memories... I can almost get hold of them, and then..." ()

The Zalkonian John Doe experienced a complete memory loss after having been severely injured in a crash. His memory loss may have been especially related to damage to his brain stem. ()

Contact with a coalescent organism could provoke a memory loss of the events that happened shortly before with the feeling of "having one's memory drained right out of one's self." ()

Dr.  Pulaski was afraid the infection with Surata IV microbes (or the remedy through triggering memories) could result in amnesia, she therefore checked Cmdr. Riker's ability to remember who he was. ()

In 2372, after his appearance and identity were changed to free him from his dishonor, Kurn (now Rodek) was told by Doctor Julian Bashir that he was suffering from a form of permanent amnesia. ()

Diagnostic
Psycho-tricorders seemed to have a subroutine for amnesia analysis as one was used to help analyze Scotty in 2267. ()

Recovery
According to a recreated Dr. Crusher in Riker's Barash-generated fantasy, memories are formed by association: it may be possible to trigger them by smells, sounds and images from the present. The idea is similar to being reminded of an old friend by listening to a song. This can be done by associational therapy, which means exposing the patient to familiar surroundings. This could stimulate the brain's neural synapses into regenerating, but the success is not very likely. ()

The memory of Kirk – suffering from amnesia due to a memory beam – was restored by a short mind meld with the Vulcan Spock. ()

Counseling was a long but promising way of restoring one's memory. In 2367, Deanna Troi assured Geordi La Forge that they would work together to restore the memories that had been deleted and replaced by the Romulans as they brainwashed him. ()

Telepathic memory retrieval was a process practiced by Ullian researchers. It involved the use of their telepathic abilities to retrieve and expand lost or faded memory engrams of an individual. It could be used to cure amnesia, but the memory could be altered by ill-willed Ullians in the case of a memory invasion. In 2368,  Jev first invaded Deanna Troi's memories causing coma and amnesia about the invasion. Then when she awoke, Jev altered the memories she asked him to retrieve in order to blame his father Tarmin for the crime. ()

Androids
Androids could be affected by memory loss. One possible cause of amnesia was an overload of the positronic net, in such case the memories from before the overload could be recovered but none of those between the accident and the reparation. () Data's programming possessed a memory loss fail-safe system to prevent anyone from taking advantage of him if this happened. (; )

Dealing with amnesia
The first thing commonly done when persons were amnesic, was asking them about the last thing they could remember, or if they could recall anything from the past. ()

One method of dealing with amnesia was waiting, hoping the memories would simply return. Especially when there was no other illness or wound. ()

Persons suffering from memory loss were often given a new name when their amnesia occurred in a place where no one knew them. On Barkon IV, Data was named Jayden by two children. The two had considered calling him "Radioactive", for they mistook the inscription on the case Data carried for his actual name. () 

Captain Kirk, who was in a similar situation on the planet Amerind, was asked his name by the Natives and struggled to remember, saying, "Ki... Ku...". He therefore was named Kirok. () 

The English placeholder name John Doe, commonly used in legal action, case or discussion for a party, whose true identity is unknown was used by Dr. Crusher to name a survivor of a crash. When John Doe recovered consciousness he could not remember who he was and kept this name. () 

As amnesics had lost their memories of events, they were sometimes considered unreliable even for information not related to their personal memory. When Data was suffering memory loss, he was not believed on Barkon IV when he said that fire was not an element. ()

Likewise, in situations in which a faked memory loss would be convenient, amnesics were often accused of lying. While investigating on the murder of Keith Rocha, Riker assumed the suspect Aquiel Uhnari was hiding information or had conveniently forgotten details. ()

Amnesia could also be hard to believe for the patient, in case the loss is not complete but only for some memories or a lapse of time. The first concern of amnesics was how to recover their memory. ()

Similar conditions

Imagined amnesia
In 2367 William T. Riker was told in a holographic projection, that a virus had caused a form of amnesia, destroying all his memories from the moment he became infected to the present, supposedly sixteen years later. Riker discovered little by little details of his life in those years, without remembering anything. He then noticed things that could not be possible, for example a photograph of a holodeck creation, who was said to have been his wife. Riker then confronted everyone with the lie and the holo-images vanished. ()

Self-imposed memory blockade
After a traumatic event  persons could create a memory block on themselves. The deletion of any traces, log entries for instance, could be practiced. Telepaths such as the Cairn could sense the blocked memories as "a part of [the person] that is dark," meaning they could not see it. Under intense fatigue and stress the blockade could break down, particularly if a similar event was witnessed. In the case of Lwaxana Troi this led to her suffering a coma. She had effectively erased all memories of her daughter Kestra after her accidental drowning and could not stand the sudden recovery of them. Only after facing the truth and accepting those memories, could Lwaxana's psyche find peace and have her awaken. ()

Vulcans had the ability of drawing out memories with the help of a pyllora: a person who, during a mind meld, acted as guide or counselor within a memory experienced, to assist the subject with repressing the memory. Tuvok repressed a traumatic memory that way, when he was infected with a memory virus. ()

Memory wipe
A memory wipe was a medical procedure that was used to selectively erase memories from an individual's mind. It could also be used to implant new memories. It was generally practiced without that person's knowledge however was not effective on all species. (; )

Types of amnesia
 Anterograde amnesia, a medical condition which prevented the sufferer from storing new memories.
 Functional amnesia
 Hysterical amnesia
 Lacunar amnesia, a psychological form of memory loss.
 Short-term memory loss

See also
 
 Repressed memory

External link
 
Medical conditions