Brass is a metal alloy made of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties. It is a substitutional alloy: atoms of the two constituents may replace each other within the same crystal structure.

By comparison, bronze is principally an alloy of copper and tin. However, bronze and brass may also include small proportions of a range of other elements including arsenic, phosphorus, aluminium, manganese, and silicon. The term is also applied to a variety of brasses, and the distinction is largely historical. Modern practice in museums and archaeology increasingly avoids both terms for historical objects in favor of the all-embracing “copper alloy”.