Source : Webster's Revised Unabridged Dictionary (1913)

Eardrum \Ear"drum`\, n. (Anat.)
   The tympanum. See Illust. of {Ear}.

Source : WordNet®

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