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]