According to Encarta, an alter ego (Latin for "the other I") can be broadly defined as "a second side to somebody's personality, different from the one that most people know." This "second side" can take a number of forms, from merely a daydream of oneself as a hero to something more tangible, like a split personality.

Psychology

The term "alter ego" has its roots in early 19-century psychology. It was originally used to describe symptoms of schizophrenia.

Alter Ego in Comics

Comics are famous for showcasing alter egos. Superman and Clark Kent, Batman and Bruce Wayne, and Spiderman and Peter Parker are all examples of famous comic book alter-ego relationships.

In Literature

Literature is filled with examples of alter-ego relationships. Fiction writers have often created central characters that are essentially fantastical or idealized versions of themselves. Writers have also created two or more characters that are alter egos of each other; a famous example of this is the relationship between Robert Louis Stephenson's Dr. Jekyll and Mr. Hyde.

Celebrity Alter Egos

Many celebrities have alter egos---some are merely stage names; others are characters that they play either on or off the screen. Hannah Montana and Miley Cyrus, The Notorious B.I.G. and Biggie Smalls, and Sacha Barron Cohen and Ali G/Borat Bruno are some examples of celebrity alter egos.

The Internet

Between social networking sites, blogs and MMORPGs (massively multiplayer online role-playing games), millions of people have online alter egos. When a person uses the Internet to create a fictionalized version of herself, it becomes a form of alter ego.

Other "Alter Egos"

Businesses and products have used the term "alter ego." "Alter Ego" is the name of an MMORPG, comic dealer, band, magazine, as well as various websites and books.

Source:

MSN Encarta: alter ego

enotes: International Dictionary of Psychoanalysis: Alter Ego

StateMaster: Encyclopedia: Alter ego

Resource:

PBS: Art:21: Lesson 3---The Alter-Ego Saves the Day

guidespot: Famous Alter Egos: What's yours???