The trombone is a kind of bugle or trumpet instrument that uses a sliding mechanism to change the length of the pipe instead of keys, and dates from the Renaissance and Baroque eras. In previous eras it was also known as the sackbutt.
It has a mouthpiece which is played with vibrating lips like the bugle or trumpet and its long length is shortened by curling it back onto itself. It is flared towards the exit, with a large bell. A valved trombone is also being made, with valves similar to those used in the trumpet. This extends the length of the pipe even further, resulting in even lower pitches.
Source: http://www.miayf.org/brasswind/trombone/
As early as the 1400s a bugle-like instrument was known that used a sliding mechanism to change the length of the pipe.