Source : Webster's Revised Unabridged Dictionary (1913)

Aptness \Apt"ness\, n.
   1. Fitness; suitableness; appropriateness; as, the aptness of
      things to their end.

            The aptness of his quotations.        --J. R. Green.

Source : WordNet®

aptness
     n 1: a disposition to behave in a certain way; "the aptness of
          iron to rust"; "the propensity of disease to spread"
          [syn: {propensity}]
     2: appropriateness; "the phrase had considerable aptness" [syn:
         {appositeness}] [ant: {inaptness}, {inaptness}]