The Collaborative International Dictionary of English v.0.48:

Fatty \Fat"ty\, a.
   Containing fat, or having the qualities of fat; greasy;
   gross; as, a fatty substance.
   [1913 Webster]

   Fatty acid (Chem.), any one of the paraffin series of
      monocarbonic acids, as formic acid, acetic, etc.; -- so
      called because the higher members, as stearic and palmitic
      acids, occur in the natural fats, and are themselves
      fatlike substances.

   Fatty clays. See under Clay.

   Fatty degeneration (Med.), a diseased condition, in which
      the oil globules, naturally present in certain organs, are
      so multiplied as gradually to destroy and replace the
      efficient parts of these organs.

   Fatty heart, Fatty liver, etc. (Med.), a heart, liver,
      etc., which have been the subjects of fatty degeneration
      or infiltration.

   Fatty infiltration (Med.), a condition in which there is an
      excessive accumulation of fat in an organ, without
      destruction of any essential parts of the latter.

   Fatty tumor (Med.), a tumor consisting of fatty or adipose
      tissue; lipoma.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fatty acid
    n 1: any of a class of aliphatic monocarboxylic acids that form
         part of a lipid molecule and can be derived from fat by
         hydrolysis; fatty acids are simple molecules built around a
         series of carbon atoms linked together in a chain of 12 to
         22 carbon atoms