Source : Webster's Revised Unabridged Dictionary (1913)

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. ?, ?,
   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.

   2. (Zo["o]l.) One of the naked, inflatable air sacs on the
      neck of the prairie chicken and other species of grouse.

   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.

   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.

Source : WordNet®

tympanum
     n 1: the main cavity of the ear; between the eardrum and the
          inner ear [syn: {middle ear}, {tympanic cavity}]
     2: the membrane in the ear that vibrates to sound [syn: {eardrum},
         {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}, {tympani}, {timpani}]
     [also: {tympana} (pl)]