Amnesia (from Greek ἀμνησία from ἀ- meaning 'without' and μνήμη memory), also known as amnesic syndrome, is a deficit in memory. The memory can be either wholly or partially lost. And it may be due to organic brain disorders like dementia or psychiatric conditions like dissociation. 

Dissociative amnesia is characterized by an inability to recall autobiographical information. This amnesia may be localized (i.e., an event or period of time), selective (i.e., a specific aspect of an event), or generalized (i.e., identity and life history). It is an inability to recall autobiographical information that is inconsistent with normal forgetting. It may or may not involve purposeful travel or bewildered wandering (i.e., fugue). 

Although some individuals with amnesia promptly notice that they have 'lost time' or that they have a gap in their memory, most individuals with dissociative disorders are initially unaware of their amnesias. For them, awareness of amnesia occurs only when personal identity is lost or when circumstances make these individuals aware that autobiographical information is missing (e.g., when they discover evidence of events they cannot recall or when others tell them or ask them about events they cannot recall). 

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