The Collaborative International Dictionary of English v.0.48:

Aptitude \Apt"i*tude\, n. [F. aptitude, LL. aptitudo, fr. L.
   aptus. See Apt, and cf. Attitude.]
   1. A natural or acquired disposition or capacity for a
      particular purpose, or tendency to a particular action or
      effect; as, oil has an aptitude to burn.
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            He seems to have had a peculiar aptitude for the
            management of irregular troops.       --Macaulay.
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   2. A general fitness or suitableness; adaptation.
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            That sociable and helpful aptitude which God
            implanted between man and woman.      --Milton.
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   3. Readiness in learning; docility; aptness.
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            He was a boy of remarkable aptitude.  --Macaulay.
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WordNet (r) 3.0 (2006):

aptitude
    n 1: inherent ability [ant: inaptitude]