a bass horn (brass wind instrument) that is the tenor of the tuba family

a low brass instrument resembling a small tuba that is rarely played outside the concert or marching band setting

The euphonium is a conical-bore, tenor-voiced brass instrument. It derives its name from the Greek word euphonos, meaning "beautiful-sounding" or "sweet-voiced" (eu means "well" or "good") and phonium means "voice"). The euphonium is a valved instrument; nearly all current models are piston valved, though rotary valved models do exist.