noun
: the intermediate portion of the ear of higher vertebrates consisting typically of a small air-filled membrane-lined chamber in the temporal bone continuous with the nasopharynx through the eustachian tube, separated from the external ear by the tympanic membrane and from the inner ear by fenestrae, and containing a chain of three ossicles that extends from the tympanic membrane to the vestibular fenestra and transmits vibrations to the inner ear — compare incus , malleus , stapes