Alter ego which combines alter, other, with ego, I, self, generally refers to someone with whom you are so close that you both do the same things, think alike, react similarly, and are, in temperament, almost mirror images of each other. Any such friend is your other I, your other self, your alter ego.

Digging a little bit into the derivation of three of our basic words, egoist, egotist and altruist has put us in touch with two important Latin roots, ego and alter, and has made it possible for us to explore with little difficulty, many other words that we derived from these roots.