Orchestration, also referred to as instrumentation, is the writing or adapting of music for performance by an orchestra, concert band or other ensemble. It includes decisions regarding which and how many instruments are playing the melody, harmony, counterpoint, percussion and any other part, the overall density of the music, and which register in which each instrument is playing.
Good orchestration is an extremely important aspect of creating good music, and may be at least as important as melody in creating a truly outstanding work. In fact, some of the greatest compositions became so because of their outstanding orchestration and in spite of their less than stellar melodies.
In the common practice period, most composers performed orchestration as an integral part of their composing, and each composer's unique orchestration style was a major feature of their work. However, in much music composed in recent decades, such as movie music and musicals, orchestration is often or mostly done by specialist orchestrators.