Merriam-Webster Collegiate Dictionary - ego

 noun  (plural ~s)  Etymology: New Latin, from Latin, I — more at i  Date: 1789  1. the self especially as contrasted with another self or the world  2.  a. ~tism 2  b. self-esteem 1  3. the one of the three divisions of the psyche in psychoanalytic theory that serves as the organized conscious mediator between the person and reality especially by functioning both in the perception of and adaptation to reality — compare id, super~  • ~less adjective