1. Another name or identity that one assumes. He's a clean-cut accountant during the week, but on the weekend he indulges in hedonistic pursuits as his alter ego "Jack."

2. A separate or different aspect or element of one's personality, identity, or psyche. For such a quiet woman, she has a rage and temper at times that is like some alter ego.

3. A close, inseparable friend of very similar attitudes and interests. My girlfriend and I are so similar, we are like each other's alter ego.

4. A person who acts as a substitute for or copy of another person; a doppelgänger. Due to his increasingly failing health, the dictator's son has been running the country for the past month, essentially as his alter ego.