instrumentation
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The specific combination of instruments used in a musical work or performance.
Instrumentation refers to which instruments are called for in a composition — the forces required to perform it. A string quartet requires two violins, viola, and cello. A standard big band requires five saxophones, four trumpets, four trombones, piano, bass, drums, and guitar. The choice of instrumentation fundamentally shapes a work's sound and character.
Instrumentation is related to but distinct from orchestration (the art of assigning musical material to instruments). A composer decides the instrumentation first (which instruments to use), then orchestrates (which instruments play which notes). Unusual instrumentations can define a work — Schubert's Trout Quintet uses the unusual combination of piano, violin, viola, cello, and double bass.
Beethoven wrote his Septet for a peculiar combination including clarinet, horn, and bassoon alongside strings — it became his most popular work during his lifetime, which irritated him enormously.