Amnesia is a memory condition in which memory is disturbed. In simple terms it is the loss of memory. This is usually temporary but can be permanent.
The causes of amnesia are organic (meaning physical damage to the brain) or functional (meaning the cause is not due to physical damage).
Organic causes include damage to the brain, through injury or disease, or use of certain drugs. Severe alcohol abuse can lead to a type of amnesia known as Korsakoff's Syndrome.
Functional causes are psychological in nature.