What is the definition of mastoid?

The mastoid is a bony prominence located behind the ear on the side of the skull. It is formed by the mastoid air cells, which are a series of interconnected cavities that are lined with mucous membrane. The mastoid air cells help to aerate the middle ear and to equalize pressure between the inner and outer ear.

The mastoid process is also the site of the mastoid foramen, which is an opening that allows for the passage of nerves and blood vessels to the inner ear. The mastoid process is named for its resemblance to the shape of a nipple.