[1] : The Collaborative International Dictionary of English v.0.48

ego \e"go\, n.; pl. egos. l., i.
   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.
   1913 webster +pjc

   2. 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.
      pjc

   3. egotism; as, a job requiring a diplomat without too much
      ego.
      pjc

   4. self-esteem; as, he has an overinflated ego.
      pjc
see also:
egos non-ego id superego 

[2] : WordNet (r) 2.0

ego
     n 1: an inflated feeling of pride in your superiority to others
          syn: egotism, self-importance
     2: your consciousness of your own identity syn: self
     3: psychoanalysis the conscious mind
see also:
egotism self-importance self 

[3] : Moby Thesaurus II by Grady Ward, 1.0

97 moby thesaurus words for "ego":
   i, i myself, alter, alter ego, alterum, anima, anima humana, atman,
   ba, better self, breath, breath of life, buddhi, coconscious,
   collective unconscious, conscience, conscious self, death instinct,
   divine breath, ego ideal, ego-id conflict, egocentricity,
   egocentrism, egoism, egoisticalness, egotism, egotisticalness,
   ethical self, foreconscious, he, heart, her, herself, him, himself,
   id, individualism, inner man, inner self, it, jiva, jivatma, khu,
   libidinal energy, libido, manes, me, mind, motive force,
   my humble self, myself, nephesh, number one, oneself, other self,
   ourselves, persona, personality, pleasure principle, pneuma,
   preconscious, primitive self, psyche, psychic apparatus, purusha,
   racial unconscious, ruach, self, self-centeredness,
   self-centerment, self-interest, selfishness, shade, shadow, she,
   soul, spirit, spiritual being, spiritus, subconscious,
   subconscious mind, subconscious self, subliminal, subliminal self,
   submerged mind, superego, the self, them, themselves, they,
   unconscious, unconscious mind, vital impulse, you, yours truly,
   yourself, yourselves

[4] : Bouvier's Law Dictionary, Revised 6th Ed (1856)

ego. i, myself. this term is used in forming genealogical tables, to 
represent the person who is the object of inquiry. 

Ego >> E Words
Ego, definition of term: Ego
ego_pag1.html

Ego Ideal >> E Words
Ego Ideal, definition of term: Ego Ideal
ego+ideal_pag1.html

Alter Ego >> A Words
Alter Ego, definition of term: Alter Ego
alter+ego_pag1.html