orchestration
The art of assigning musical material to specific instruments in an ensemble.
In Depth
Orchestration is the craft of deciding which instruments play which notes — transforming a musical idea from abstract pitches into specific instrumental colours. A melody played by a flute has an entirely different character from the same melody on a cello, oboe, or French horn. The orchestrator exploits these differences to create the desired sonic effect.
Ravel is widely considered the greatest orchestrator, with a seemingly magical ability to blend instrumental colours. His orchestration of Mussorgsky's Pictures at an Exhibition is studied by every composition student. Rimsky-Korsakov literally wrote the textbook (Principles of Orchestration), and his student Stravinsky revolutionised orchestral colour in The Rite of Spring.
Ravel was such a master of orchestration that Stravinsky called him a Swiss watchmaker — every instrumental choice was perfectly calibrated, with no wasted notes.