a brass instrument consisting of a long tube whose length can be varied by a U-shaped slide
source: WordNet 3.0
A powerful brass instrument of the trumpet kind, thought by some to be the ancient sackbut, consisting of a tube in three parts, bent twice upon itself and ending in a bell. The middle part, bent double, slips into the outer parts, as in a telescope, so that by change of the vibrating length any tone within the compass of the instrument (which may be bass or tenor or alto or even, in rare instances, soprano) is commanded. It is the only member of the family of wind instruments whose scale, both diatonic and chromatic, is complete without the aid of keys or pistons, and which can slide from note to note as smoothly as the human voice or a violin. Softly blown, it has a rich and mellow sound, which becomes harsh and blatant when the tones are forced; used with discretion, its effect is often solemn and majestic.