An orchestra  is a large instrumental ensemble typical of classical music, which mixes instruments from different families, including bowed string instruments such as violin, viola, cello and double bass, as well as brass, woodwinds, and percussion instruments, each grouped in sections. Other instruments such as the piano and celesta may sometimes appear in a fifth keyboard section or may stand alone, as may the concert harp and, for performances of some modern compositions, electronic instruments. A full-size orchestra may sometimes be called a symphony orchestra or philharmonic orchestra. The actual number of musicians employed in a given performance may vary from seventy to over one hundred musicians, depending on the work being played and the size of the venue. The term chamber orchestra (and sometimes concert orchestra) usually refers to smaller-sized ensembles of about fifty musicians or fewer. Orchestras that specialize in the Baroque music of, for example, Johann Sebastian Bach and George Frideric Handel, or Classical repertoire, such as that of Haydn and Mozart, tend to be smaller than orchestras performing a Romantic music repertoire, such as the symphonies of Johannes Brahms. The typical orchestra grew in size throughout the 18th and 19th centuries, reaching a peak with the large orchestras (of as many as 120 players) called for in the works of Richard Wagner, and later, Gustav Mahler.

Orchestras are usually led by a conductor who directs the performance with movements of the hands and arms, often made easier for the musicians to see by use of a conductor’s baton. The conductor unifies the orchestra, sets the tempo and shapes the sound of the ensemble. The conductor also prepares the orchestra by leading rehearsals before the public concert, in which the conductor provides instructions to the musicians on their interpretation of the music being performed. Amateur orchestras include those made up of students from an elementary school or a high school, youth orchestras, and community orchestras; the latter two typically being made up of amateur musicians from a particular city or region. The typical symphony orchestra consists of four groups of related musical instruments called the woodwinds, brass, percussion, and strings (violin, viola, cello and double bass). Other instruments such as the piano and celesta may sometimes be grouped into a fifth section such as a keyboard section or may stand alone, as may the concert harp and electric and electronic instruments. The orchestra, depending on the size, contains almost all of the standard instruments in each group.

In the history of the orchestra, its instrumentation has been expanded over time, often agreed to have been standardized by the classical period and Ludwig van Beethoven’s influence on the classical model. In the 20th century, new repertory demands expanded the instrumentation of the orchestra, resulting in a flexible use of the classical-model instruments and newly developed electric and electronic instruments in various combinations.