1. Pertaining to that scale of Greek music which proceeded by quarter tones and major thirds.
  2. Pertaining to, or concerned with, intervals smaller than a semitone; especially with reference to the interval between those notes (belonging to different keys) which in instruments of equal temperament are rendered by the same tone: e.g. between G# and Ab.

With apologies to Webster 1913, the Oxford Universal Dictionary on Historical Principles, third edition, has a more complete definition of this term.