Source : Webster's Revised Unabridged Dictionary (1913)

Subtlety \Sub"tle*ty\, n.; pl. {Subtleties}. [OE. sotelte,
   sutilte, OF. sotillete, L. subtilitas. See {Subtle}, and cf.
   {Subtility}.]
   1. The quality or state of being subtle, or sly; cunning;
      craftiness; artfulness.

            The fox which lives by subtlety.      --Shak.

   2. Nice discernment with delicacy of mental action; nicety of
      discrimination.

   3. Something that is sly, crafty, or delusive.

            Unlearned in the world's false subtleties. --Shak.

Source : WordNet®

subtlety
     n 1: a subtle difference in meaning or opinion or attitude;
          "without understanding the finer nuances you can't enjoy
          the humor"; "don't argue about shades of meaning" [syn:
          {nuance}, {nicety}, {shade}, {refinement}]
     2: the quality of being difficult to detect or analyze; "you
        had to admire the subtlety of the distinctions he drew"
        [syn: {niceness}]