The Ego is in the Freudian theory the aspect of the personality involved in self-preservation activities, and directing instinctual drives (the id) into appropriate social channels.
This is one of the three structures of the mind as conceived by psychoanalysis which obeys the "reality principle," mediating between the real world and the desires of the id.
Other /More definition:
ego is a term in Psychoanalysis which refers to the psychic entity responsible for finding realistic and practical ways to satisfy Id drives.