Source : Webster's Revised Unabridged Dictionary (1913)

Mastoid \Mas"toid\, a. [Gr. ?; ? the breast + ? form: cf. F.
   masto["i]de.] (Anat.)
   (a) Resembling the nipple or the breast; -- applied
       specifically to a process of the temporal bone behind the
       ear.
   (b) Pertaining to, or in the region of, the mastoid process;
       mastoidal.

Source : WordNet®

mastoid
     adj : relating to or resembling a nipple [syn: {mastoidal}]

mastoid
     n : process of the temporal bone behind the ear at the base of
         the skull [syn: {mastoid process}, {mastoid bone}, {mastoidal}]