The Collaborative International Dictionary of English v.0.48:

Aptness \Apt"ness\, n.
   1. Fitness; suitableness; appropriateness; as, the aptness of
      things to their end.
      [1913 Webster]

            The aptness of his quotations.        --J. R. Green.
      [1913 Webster]

   2. Disposition of the mind; propensity; as, the aptness of
      men to follow example.
      [1913 Webster]

   3. Quickness of apprehension; readiness in learning;
      docility; as, an aptness to learn is more observable in
      some children than in others.
      [1913 Webster]

   4. Proneness; tendency; as, the aptness of iron to rust.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "aptness":
   appositeness, appropriateness, bent, bump, expediency, faculty,
   fitness, flair, genius, head, helpfulness, knack, meetness, order,
   propitiousness, propriety, rightness, suitability, suitableness,
   talent, turn