mastoid

1. Of or relating to the mastoid process.
2. Shaped like a breast or nipple.

[New Latin mastoīdēs, nipple-like, mastoid (from its shape), from Greek mastoeidēs : mastos, breast + -oeidēs, -oid.]

1. the protuberance of the temporal bone located directly behind the external ear. adj 2. breastshaped.

The mastoid process.

1. Of or relating to the mastoid process, antrum, or sinuses.
2. Shaped like a breast or nipple.

The noun has one meaning:

Meaning #1: process of the temporal bone behind the ear at the base of the skull
  Synonyms: mastoid process, mastoidal

The adjective mastoid has one meaning:

Meaning #1: relating to or resembling a nipple
  Synonym: mastoidal
  Pertains to noun: nipple (meaning #1)