The term 'Music with a Number' refers to a type of musical composition known as an Opus. An Opus is a piece of music that is typically identified by a number, usually assigned in chronological order of composition. It is often used to refer to a large-scale work, such as a symphony or opera, but can also be used to refer to smaller works such as sonatas or concertos. Opus numbers are often used to distinguish between different versions of the same work, and can also be used to identify works by the same composer.