The Collaborative International Dictionary of English v.0.48:

Strikeout \Strike"out\, n. (Baseball)
   an out made by a batter against whom three strikes were
   called in a single at-bat; also, the same out as credited to
   the pitcher, in the scoring of the game.
   [PJC]

WordNet (r) 3.0 (2006):

strikeout
    n 1: an out resulting from the batter getting three strikes