Timpani is the Italian word for kettle-drums, which are tuneable drums used in the symphony orchestra.
The tension of the skin membrane can be adjusted by the player, to tune each drum to a particular bass note.
There are usually three kettle drums in the orchestra, tuned to the tonic, dominant and sub-dominant notes for the key of the music being played.
There are 4 main sizes of kettle-drum, but the middle two (26 inch and 29 inch) are the most used.