Merriam-Webster Collegiate Dictionary - mastoid

 I. adjective  Etymology: New Latin ~es resembling a nipple, ~, from Greek mastoeides, from mastos breast  Date: 1732  1. being the process of the temporal bone behind the ear; also being any of several bony elements that occupy a similar position in the skull of lower vertebrates  2. of, relating to, or occurring in the region of the ~ process  II. noun  Date: 1840 a ~ bone or process