Alter ego is a Latin expression meaning a second self, a second personality or persona within a person. The term started to be frequently used in early nineteenth century when dissociative identity disorder was first described by psychologists. An alter ego person has a double life and a double identity.
Alter-ego is also used to refer to the different behaviors any person may display in various situations. Related concepts include avatar, impersonator, and split personality.
A distinct meaning for alter-ego can be found in literary analysis, wherein it describes characters in different works who are psychologically similar, or a fictional character whose behavior, speech or thoughts intentionally represent those of the author.