A kettledrum is a very large drum typically consisting of a drum head stretched across a copper bowl. Most large orchestras include kettledrums in their percussion section.
A typical set of kettledrums has four different sizes of drums, but some orchestras have eight or more. If you've ever noticed a sound like thunder in a classical piece of music, you're probably hearing the kettledrum. The word comes from the kettle-like shape of the drum's bowl, and kettledrums are also commonly called timpani.