The Collaborative International Dictionary of English v.0.48:

Mammilla \Mam*mil"la\, n.; pl. Mammilae. [L., dim. of mamma a
   breast.] (Anat.)
   The nipple.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mammilla
    n 1: the small projection of a mammary gland [syn: nipple,
         mammilla, mamilla, pap, teat, tit]