the part of the ear including the tympanic membrane and the adjacent cavity containing the three small bones (hammer, anvil, and stirrup) that transmit the vibrations of the tympanic membrane to the inner ear
See middle ear in American Heritage Dictionary 4
noun
The space between the eardrum and the inner ear that contains the three auditory ossicles, which convey vibrations through the oval window to the cochlea. Also called tympanum.