1.nounA valved brass instrument, a sax horn, the tenor of the tuba family of instruments, having the appearance very similar to that of a tuba. It is similar to and often used instead of a "Baritone" horn. (A true Baritone has a cylindrical tubing, while the Euphonium tubing is conical, though they both cover the same range of tones. This relationship is also between the Trumpet [cylindrical] and Cornet [conical] respectively.)