The Collaborative International Dictionary of English v.0.48:

Tam-tam \Tam"-tam`\, n. [Hind.; of imitative origin.] (Mus.)
   (a) A kind of drum used in the East Indies and other Oriental
       countries; -- called also tom-tom.
   (b) A gong. See Gong, n., 1.
       [1913 Webster]

WordNet (r) 3.0 (2006):

tam-tam
    n 1: a percussion instrument consisting of a metal plate that is
         struck with a softheaded drumstick [syn: gong, tam-tam]