Loss (at least partial) of memory. It has several forms. Amnesia can be permanent or temporary, global or selective. It can be retrograde: the memory of events before the cause of the amnesia is deleted. It can be anterograde (or post-traumatic): loss of memory or difficulty remembering events after the cause that triggered the amnesia.
It has various causes. Chronic alcoholism can cause difficulty remembering recent events (Korsakoff syndrome) as can some medicines including vallium.
A head injury may cause amnesia (anterograde or retrograde). The duration of events affected can be limited to a few minutes after the injury but may extend to days or even months before and afterwards.
Transient global amnesia (generally lasting a few hours) can be caused by thermal shock (immersion in cold or hot water), emotional shock, violent physical exercise or sexual intercourse.
Amnesia can be psychogenic and psychological in origin. It can be selective, involving events that are difficult for the person to confront (for example physical violence to other people). It can be prolonged and global. The person can then forget things even their name.