1.NounAn individual's alternate personality or persona; another self.
"Clark Kent's coworkers finally discovered his alter ego, Superman."
2.NounA very close and intimate friend.
3.NounA corporation used by a person to conduct personal business in an attempt to shield himself or herself from personal liability, and which a court may penetrate by "piercing the corporate veil" to impose liability on the person when they commit fraud or injustice.