What is an Orchestra?
The term “ORCHESTRA” has many meanings and can evoke a variety of associations. Even a traditional symphony orchestra is regarded by some people as a collection of instruments and players, while others consider the orchestra to be an instrument in itself. There are good arguments for both viewpoints. On the one hand, a great number of musicians and diverse types of instruments are involved. On the other hand, what primarily impresses the composer, the concert audience may not be the individuals and their parts of the music, but rather how they unite as a tonal, harmonious, melodic whole.
If we ask a member of the orchestra to play his part alone for half a minute, the music is often unrecognizable. The fragment might be taken from one of our favorite symphonies, yet it can sound add and vague. In much music, only one or two parts at a time produce the melodies we remember; the other parts are busy building up harmonies, accentuating rhythms, and filling out the tonal palette. Hearing them separately makes it hard to understand their context.
A detached melody part, too, occasionally sound strange. Most music lovers are familiar with the beginning of the finale in Tchaikovsky’s Pathetique symphony. But actually the violins play quite different melodies. Only when they are heard together do we realize that the combination is the well known, continuous melody line.

Thus, we have strong reasons for thinking of the orchestra as a single instrument. An obvious parallel is a large organ, whose distinctive stops—the registers—nobody would call it a separate instrument. Strictly speaking, every pipe in the organ is its own instrument!
The organ, of course, is played by one person. Some of us may want to speak of an orchestra only when several musicians participate. As an extreme case, take the street-bucker with a harmonica strapped in front of his mouth, a guitar in his hand, cymbals between his elbow and body, and a bass drum on his back with a cord to his foot. Is this an orchestra? We might insist that four different musicians would have to be playing the instruments. Nonetheless, it has been termed a “one-man orchestra”. Whatever our attitude, the above examples illustrate the difficulty of finding clear definitions.

The Restaurant Trio
Things become even more complicated if we compare orchestras for “serious” music with those for entertainment, dance and jazz. To call a string quartet, quintet or sextet an orchestra would probably never occur to us. Such groups are known as chamber ensembles. Yet in the genres of “light”orchestra is simply a group—large or small—of musicians who play together. Even the restaurant trios of yesteryear could be commanded as follows: "Headwaiter, tell the orchestra to play Schubert’s military march!"

The Question of Size
Fifteen musicians seem a lot for a jazz or dance orchestra, here, the common term is a “big band”. But if a classical orchestra includes the same number of players, it is called a chamber orchestra, for the opposite purpose of emphasizing its small format. Indeed, a chamber orchestra with fifteen musicians is almost tiny!
So where does the border run between symphony and chamber orchestra? Once again, no strict rule exists. At times, it even depends on the kind of music being played. Many of Haydn’s symphonies are written for a group of bowed instruments along with two oboes, two horns, and occasionally a couple of kettledrums. This is certainly a symphony orchestra, in fact the very prototype of one. It’s the amount of 20-25 musicians, though, that constitutes a chamber orchestra by modern standards.

The Ensemble
A chamber orchestra can be more easily distinguished from a chamber ensemble. Frequently an ensemble is defined as a group in which every musician performs an individual part, whereas some parts in an orchestra are choral: the same part is played by several musicians, like the singers in a choir. For example, Schubert’s Octet gives separate music to each player, but his symphonies—which may themselves resemble chamber music—present the string parts chorally to balance the wind instruments.

A Title With Traditions
In sum, it is impossible to unambiguously state what an orchestra is, or distinguish it from other musical groups. The concept of an orchestra is elusive, and we should instead focus upon the word, originally this had nothing to do with musicians or their instruments. In ancient Greece, an orchestra meant the forepart of an amphitheatre’s stage, or the area between the public and the rounded front edge of the stage. It was there that the chorus acted and danced. The Greek word Khoros also meant a place for dancing. Hence, the words “chorus” and “orchestra” have a common background.

Did the Greek organize musical groups that were similar to later ensembles or orchestras? Presumably not. Their main stringed instrument, the soft kithara, and their sharp oboe-like aulos were too different to sound well together. Still more important, the two kinds of instruments contrasted in symbolic and religious significance. To combine them was, therefore, hardly desirable.

As late as the 17th century, an orchestra was the place in front of the scene where musicians sat to accompany operas. Not before some way into the 1700’s did musical groups begin to be termed orchestra, both in the theatre and elsewhere.