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".
An alter ego can be thought of as a person's clone or second self. A professional alter ego might be a trusted aide who knows exactly what the boss wants done. A personal alter ego might be a close friend who is almost like a twin. Alter ego can also refer to the second, hidden side of one's own self.