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 nineteenth centuries, but changed very little in composition during the course of the twentieth century.
Orchestral music, by extension, is any form of music that is composed for performance by an orchestra. The word is not so much a name for a genre as it is an umbrella term for any form of music that is meant to be performed by a group of musicians utilizing various different instruments from the string, brass, woodwind and percussion families.
The word "orchestral" is often used interchangeably with "symphonic" as well as "classical," though technically speaking, the latter of those is incorrect, as "classical music" refers specifically to music composed during the classical period (mid-1700s through early 1800s). .