ego

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The self, especially as distinct from the world and other selves.
• noun In psychoanalysis, the division of the psyche that is conscious, most immediately controls thought and behavior, and is most in touch with external reality.
• noun An exaggerated sense of self-importance; conceit.
• noun Appropriate pride in oneself; self-esteem.

from The Century Dictionary.

• noun The “I”; that which feels, acts, and thinks; any person's “self,” considered as essentially the same in all persons. This use of the word was introduced by Descartes, and has long been current in general literature.

from the GNU version of the Collaborative International Dictionary of English.

• noun The conscious and permanent subject of all psychical experiences, whether held to be directly known or the product of reflective thought; the subject consciously considered as “I” by a person; -- opposed to non-ego.
• noun (Psychoanalysis) that one of the three parts of a person's psychic apparatus that mediates consciously between the drives of the id and the realities of the external physical and social environment, by integrating perceptions of the external world and organizing the reactions to it. Contrasted with the id and superego.
• noun egotism.
• noun self-esteem.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun the self, especially with a sense of self-importance
• noun psychology, Freudian the most central part of the mind, which mediates with one's surroundings

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun your consciousness of your own identity
• noun (psychoanalysis) the conscious mind
• noun an inflated feeling of pride in your superiority to others