The Collaborative International Dictionary of English v.0.48:

Kettledrum \Ket"tle*drum`\ (-dr[u^]m`), n.
   1. (Mus.) A drum made of thin copper in the form of a
      hemispherical kettle, with parchment stretched over the
      mouth of it.
      [1913 Webster]

   Note: Kettledrums, in pairs, were formerly used in martial
         music for cavalry, but are now chiefly confined to
         orchestras, where they are called tympani.
         [1913 Webster]

   2. An informal social party at which a light collation is
      offered, held in the afternoon or early evening. Cf.
      Drum, n., 4 and 5.
      [1913 Webster]

WordNet (r) 3.0 (2006):

kettledrum
    n 1: 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]