An orchestra is a sizable instrumental ensemble that contains sections of string, brass, woodwind, and percussion instruments. The term orchestra derives from the Greek ορχήστρα, the name for the area in front of an ancient Greek stage reserved for the Greek chorus. The orchestra grew by accretion throughout the eighteenth and 19th centuries, but changed very little in composition during the course of the 20th century.

A smaller-sized orchestra for this time period (of about fifty players or fewer) is called a chamber orchestra. A full-size orchestra (about 100 players) may sometimes be called a "symphony orchestra" or "philharmonic orchestra"; these modifiers do not necessarily indicate any strict difference in either the instrumental constitution or role of the orchestra, but can be useful to distinguish different ensembles based in the same city (for instance, the London Symphony Orchestra and the London Philharmonic Orchestra). A symphony orchestra will usually have over eighty musicians on its roster, in some cases over a hundred, but the actual number of musicians employed in a particular performance may vary according to the work being played and the size of the venue. A leading chamber orchestra might employ as many as fifty musicians; some are much smaller than that. Orchestras can also be found in schools.