The Collaborative International Dictionary of English v.0.48:

Tympanum \Tym"pa*num\, n.; pl. E. Tympanums, L. Tympana.
   [L., a kettledrum, a drum or wheel in machines, the
   triangular area in a pediment, the panel of a door, Gr.
   ty`mpanon, ty`panon, fr. ? to strike, beat. See Type, and
   cf. Timbrel.]
   1. (Anat.)
      (a) The ear drum, or middle ear. Sometimes applied
          incorrectly to the tympanic membrane. See Ear.
      (b) A chamber in the anterior part of the syrinx of birds.
          [1913 Webster]

   2. (Zool.) One of the naked, inflatable air sacs on the neck
      of the prairie chicken and other species of grouse.
      [1913 Webster]

   3. (Arch.)
      (a) The recessed face of a pediment within the frame made
          by the upper and lower cornices, being usually a
          triangular space or table.
      (b) The space within an arch, and above a lintel or a
          subordinate arch, spanning the opening below the arch.
          [1913 Webster]

   4. (Mech.) A drum-shaped wheel with spirally curved
      partitions by which water is raised to the axis when the
      wheel revolves with the lower part of the circumference
      submerged, -- used for raising water, as for irrigation.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tympanum
    n 1: the main cavity of the ear; between the eardrum and the
         inner ear [syn: middle ear, tympanic cavity,
         tympanum]
    2: the membrane in the ear that vibrates to sound [syn:
       eardrum, tympanum, tympanic membrane, myringa]
    3: a large hemispherical brass or copper percussion instrument
       with a drumhead that can be tuned by adjusting the tension on
       it [syn: kettle, kettledrum, tympanum, tympani,
       timpani]

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "tympanum":
   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, bongo drum,
   bony labyrinth, cauliflower ear, cochlea, conch, concha, conga,
   conjuctiva, drum, drumhead, drumskin, drumstick, ear, ear lobe,
   eardrum, endolymph, external ear, hammer, hymen, incus, inner ear,
   jazz stick, kettle, kettledrum, lobe, lobule, lug, maidenhead,
   malleus, mastoid process, membrana serosa, membrana tympana,
   membrane, membranophone, 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, side drum,
   snare, snare drum, stapes, stirrup, tabor, taboret, tabret,
   tam-tam, tambourine, tenor drum, timbrel, timpani, tom-tom,
   troll-drum, tymp stick, tympan, tympanic cavity, tympanic membrane,
   tympanon, velum, vestibule, war drum