noun (n)

• a disposition to behave in a certain way(noun.attribute)

  Example:
  The aptness of iron to rust.
  

  Synonym:
  propensity
  

  source: wordnet30
• appropriateness for the occasion(noun.attribute)

  Example:
  The phrase had considerable aptness.
  

  Synonym:
  appositeness
  Antonym:
  inappositeness, inaptness
  

  source: wordnet30
• Fitness; suitableness; appropriateness; as, the aptness of things to their end.(noun)

  source: webster1913