Amnesia, is a form of memory loss in which a person loses some or all recollection of themselves and their past. In video games, it is a commonly used plot device in which the premise is set-up by way of the player taking control of an amnesiac protagonist.
Developers use this conceit to explain who the character really is throughout the story without having to use a lot of exposition up front. Over the course of the game, the protagonist's memory is either restored piecemeal or all at once, filling blanks in the story.
In reality, there are different forms of amnesia. Retrograde amnesia is the form most commonly depicted in fiction; patients that suffer from it have lost their memory of some or all of their past. Anterograde amnesia, on the other hand, inhibits the patient's ability to create and retain new memories. Their minds in essence continually reset to a state in which they cannot remember anything beyond a specific point in their lives.
Examples in Video Games
Psychic City / Hoshi wo Miru Hito
In the 1984 cyberpunk role-playing game Psychic City, and its 1987 NES console remake Hoshi wo Miru Hito, the protagonist is a psychic who has no memory of who he is. He soon discovers that he, along with other fellow psychics, are being pursued, due to the city's inhabitants being brainwashed into believing that psychics are a threat to humanity. It is later revealed that a supercomputer has been controlling the minds of the city's inhabitants and is intent on capturing psychics.
Deja Vu: A Nightmare Comes True
In the graphic adventure title Deja Vu, the player character starts the game off by awakening in a toilet stall with his memory completely wiped. He soon finds himself embroiled in a murder mystery, in which he appears to be a prime suspect. When he finally regains his memory, the protagonist remembers that he is Ace Harding, a private investigator, and he had been drugged into an amnesiac state and framed for the murder.
The Magic of Scheherazade
The 1987 NES role-playing game The Magic of Scheherazade begins when one day a mass of demons suddenly attack Arabia at the evil magician Sabaron's command. The hero attempts to stop the assault, but fails miserably. As a result, his sweetheart Scheherazade is taken from him, he is flung into a different time period, and his memories are erased. With the help of his guide Coronya, the hero goes on a quest to regain his lost memories, return to his time period, and rescue Scheherazade. When he eventually regains his memories, he realizes that she was never kidnapped, but was accompanying him all along disguised as Coronya.