The Collaborative International Dictionary of English v.0.48:

Mastoid \Mas"toid\, a. [Gr. ?; masto`s the breast + e'i^dos
   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.
       [1913 Webster]

WordNet (r) 3.0 (2006):

mastoid
    adj 1: of or relating to or in the region of the mastoid process
    2: relating to or resembling a nipple [syn: mastoid,
       mastoidal]
    n 1: process of the temporal bone behind the ear at the base of
         the skull [syn: mastoid, mastoid process, mastoid
         bone, mastoidal]