A bass horn (brass wind instrument) that is the tenor of the tuba family WordNet 3.0 & Open English WordNet

The euphonium is a medium-sized, 3 or 4-valve, often compensating, conical-bore, tenor-voiced brass instrument that derives its name from the Ancient Greek word εὔφωνος euphōnos, meaning "well-sounding" or "sweet-voiced". Wikipedia

A conical-bore, tenor-voiced brass instrument. OmegaWiki

A 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.). Wiktionary

Brass instrument. Wiktionary (translation)