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]