Alter ego (Latin which means "I am the other") is the second self that is believed to be different than the average person or the real personality. This term was used in the early nineteenth century when personality breakdown disorders were first explained by psychologists . Someone who has Alter ego is said to live a double life.
A different meaning from Alter ego can be found in literary analysis, which depicts characters in different works that are psychologically similar, or fictional characters whose behavior, speech, or mind deliberately represent the author  or with other types of appearance.
The existence of "another self" was first recognized in the 1730s. Anton Mesmerm uses hypnosis to separate Alter ego. This experiment shows a different pattern of behavior from the individual's personality in a conscious state than when hypnotized. Other characters are developed in changing consciousness but in the same body .
Alter ego is also used to refer to different behaviors of each person displayed in certain circumstances
Alter ego is just like another personality of an individual. A distinct character from the normal character known with the individual.
It is like a Jekyll and Hyde situation. The user is good at certain times and totally different in other instance
An alter ego is another ego or another personality that one can have, or like a secondary one. It's like being yourself 80% of time and becoming someone totally different in certain situations.
An alter ego is a second personality that lives within the same person. A person who leads a double life, with a completely different career, family, or personality, is said to have an alter ego. Most superheroes are alter egos of regular people.
The Encyclopedia Britannica says Sigmund Freud first coined the term "ego" to mean the portion of the human personality that is experienced as the “self” or “I” and is in contact with the external world through perception.
The Alter Ego
From Latin, literally meaning, "second I," the alter ego is a separate and distinct personality created by the individual--usually to live out a better, or fantasy, version of the self.
The Merriam-Webster Online Dictionary says the noun alter ego means a second self, as in a trusted friend--the opposite side of a personality, or a counterpart.
In the analysis of literature, the term alter ego can be used to describe characters that are psychologically identical, or characters who intentionally represent the behaviors, thoughts or ideas of the author.
Famous Alter Egos
Some of the most famous alter egos are Dr. Jekyll for Mr. Hyde, Superman for Clark Kent, Charlie Brown for Charles Schultz and Hannah Montana for Miley Cyrus.