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]