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: Webster's Revised Unabridged Dictionary (1913)