The Collaborative International Dictionary of English v.0.48:

Eardrum \Ear"drum`\, n. (Anat.)
   The tympanum. See Illust. of Ear.
   [1913 Webster]

WordNet (r) 3.0 (2006):

eardrum
    n 1: the membrane in the ear that vibrates to sound [syn:
         eardrum, tympanum, tympanic membrane, myringa]

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "eardrum":
   Eustachian tube, allantoic membrane, amnion, amniotic sac, anvil,
   arachnoid membrane, auditory apparatus, auditory canal,
   auditory meatus, auditory nerve, auditory ossicles, auditory tube,
   auricle, basement membrane, basilar membrane, bony labyrinth,
   cauliflower ear, cochlea, conch, concha, conjuctiva, drumhead, ear,
   ear lobe, endolymph, external ear, hammer, hymen, incus, inner ear,
   lobe, lobule, lug, maidenhead, malleus, mastoid process,
   membrana serosa, membrana tympana, membrane, meninges, meninx,
   middle ear, neurilemma, organ of Corti, outer ear, oval window,
   pellicle, pericardium, perilymph, perineurium, pinna, pleura,
   round window, secondary eardrum, semicircular canals, serosa,
   shell, stapes, stirrup, tympanic cavity, tympanic membrane,
   tympanum, velum, vestibule