Tam-tam n. : A kind of drum used in the East Indies and other Oriental countries; -- called also tom-tom.

Tam-tam n. : A gong. See Gong, n., 1.

Example:
What's the difference between a tom tom [drum] and a tam tam [also percussion of some type]?

Taxonomy
gong, tam-tam, chau gong, instruments, rim, tone, bell, tam-tams, song, metal, boss, Opera, signal, bowl, symphony orchestra, diameter, nipple gong, free encyclopedia.

Defination from the web:
tam-tams are now used at Bayreuth in Parsifal to reinforce the bell instruments, although there is no indication given in the score. ... The Moody Blues' landmark album Days of Future Passed opens with a crescendo roll on tam-tam, and closes with a single stroke which fades to silence. ...