Source : Webster's Revised Unabridged Dictionary (1913)

Ego \E"go\, n. [L., I.] (Met.)
   The conscious and permanent subject of all psychical
   experiences, whether held to be directly known or the product
   of reflective thought; -- opposed to non-ego.

Source : WordNet®

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