liveabout.com によると、
The word “orchestra” was used to describe the place where musicians and dancers performed in ancient Greece. The orchestra, or symphony orchestra, is generally defined as an ensemble mainly composing of bowed stringed instruments, percussion, wind and brass instruments. Often, the orchestra is composed of 100 musicians and may be accompanied by a chorus or be purely instrumental. In today’s setting, the word “orchestra” not only pertains to a group of musicians but also to the main floor of a theater.
ついでに、Oxford languagesによると、
orchestraの語源（ORIGIN）は、
early 17th century: via Latin from Greek orkhēstra, from orkheisthai ‘to dance’. とあるので、元は「おどる」という意味だったようです
その派生語も下に紹介します。
orchestrate /ˈɔːkɪstreɪt/ verb
1. arrange or score (music) for orchestral performance. Synonym:arrange
2. plan or coordinate the elements of (a situation) to produce a desired effect, especially surreptitiously. Synonym: organize
そして、
orchestration /ɔːkɪˈstreɪʃ(ə)n/ noun
1. the arrangement or scoring of music for orchestral performance.”Prokofiev’s mastery of orchestration”
2. the planning or coordination of the elements of a situation to produce a desired effect, especially surreptitiously.”the orchestration of the campaign needed tightening”