What is Alter Ego?

Legal Definition
An alter ego (Latin, "the other I") is a second self, which is believed to be distinct from a person's normal or original personality. A person who has an alter ego is said to lead a double life. The term appeared in common usage in the early 19th century when dissociative identity disorder was first described by psychologists. Cicero first coined the term as part of his philosophical construct in 1st century Rome, but he described it as "a second self, a trusted friend".

A distinct meaning of 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. It's also used to designate the best friend of another character in a story. Similarly, the term alter ego may be applied to the role or persona taken on by an actor or by other types of performers.

The existence of "another self" was first recognized in the 1730s. Anton Mesmer used hypnosis to separate the alter ego. These experiments showed a behavior pattern that was distinct from the personality of the individual when he was in the waking state compared to when he was under hypnosis. Another character had developed in the altered state of consciousness but in the same body.

Alter ego is also used to refer to the different behaviors any person may display in certain situations. Related concepts include avatar, doppelgänger, impersonator, and dissociative identity disorder (DID).
-- Wikipedia
Legal Definition
Legal doctrine whereby the court finds a corporation lacks a separate identity from an individual or corporate shareholder, resulting in injustice to the corporation’s debtors. Finding alter ego gives the court cause to pierce the corporate veil and hold individual shareholders personally liable for debts of the corporation.
Illustrative caselaw
See, e.g. Fletcher v. Atex, Inc., 68 F.3d 1451 (1995) and United States v. Bestfoods, 524 US 51 (1998).
See also
Legal Definition
A document that holds stockholders and directors responsible for loss. This is used when the company is a front. It is not the independent entity imagined by corporate law.
Legal Definition
"Another I". A second identity living within a person.