NOUN (2)

1. the first and last; signifies God's eternity;

2. the basic meaning of something; the crucial part;

The Collaborative International Dictionary of English v.0.48:

omega \o*me"ga\ ([=o]*m[=e]"g[.a] or [=o]*m[=a]"g[.a] or
   [=o]"m[-e]*g[.a]; 277), n. [NL., fr. Gr. 'w^ me`ga, i.e., the
   great or long o. Cf. Mickle.]
   1. The last letter of the Greek alphabet. See Alpha.
      [1913 Webster]

   2. The last; the end; hence, death.
      [1913 Webster]

            "Omega! thou art Lord," they said.    --Tennyson.
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   Alpha and Omega, the beginning and the ending; hence, the
      chief, the whole. --Rev. i. 8.
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            The alpha and omega of science.       --Sir J.
                                                  Herschel.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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