Source : Webster's Revised Unabridged Dictionary (1913)

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.

            He seems to have had a peculiar aptitude for the
            management of irregular troops.       --Macaulay.

   2. A general fitness or suitableness; adaptation.

            That sociable and helpful aptitude which God
            implanted between man and woman.      --Milton.

   3. Readiness in learning; docility; aptness.

            He was a boy of remarkable aptitude.  --Macaulay.

Source : WordNet®

aptitude
     n : inherent ability [ant: {inaptitude}]