Source : Webster's Revised Unabridged Dictionary (1913)

Omega \O*me"ga\, n. [NL., fr. Gr. ?, i.e., the great or long o.
   Cf. {Mickle}.]
   1. The last letter of the Greek alphabet. See {Alpha}.

   2. The last; the end; hence, death.

            ``Omega! thou art Lord,'' they said.  --Tennyson.

   {Alpha and Omega}, the beginning and the ending; hence, the
      chief, the whole. --Rev. i. 8.

            The alpha and omega of science.       --Sir J.
                                                  Herschel.

Source : WordNet®

alpha and omega
     n 1: the first and last; signifies God's eternity
     2: the basic meaning of something; the crucial part