Orchestration
Distribution of the parts among instruments.
What it means
Orchestration is the art of distributing musical ideas among the instruments of an orchestra. This involves more than assigning parts: it concerns register, doubling, mixture, balance, and which colour a line is to carry. The term is also used for the arrangement of an existing piece, for instance when a piano work is set for orchestra. Instrumentation is often used with the same meaning.
Musical use
The effect of a work depends substantially on this. The same melody sounds different from the oboe than from the solo violin, and an octave doubling changes not only the volume but the colour. Among the basic questions are: which instruments carry in which register, where do gaps in the sound arise, how does a middle voice remain audible, when does one mix and when does one set colours against each other. Since the nineteenth century, orchestration has been a subject of teaching in its own right with an elaborated literature; at the same time it remains bound to practical knowledge of the instruments — their ranges, fingerings, breathing needs, and tricky registers.
Performance practice
Whoever orchestrates should know the instruments at close hand and, in case of doubt, ask players; much of what works on paper is awkward or sounds different than intended. Notate transposing instruments correctly and give sounding pitches where misunderstandings are possible. Plan in breathing places and rests, especially for winds and high brass parts. Check balance not at the piano but by listening to a rehearsal. Keep parts legible and page-turns practicable.
Bear in mind that legibility and playability belong to the work: a part with impossible page-turns, missing bar numbers, or unclear transposition indications costs rehearsal time, even if the music is well set. Check the parts before handing them over. Rimsky-Korsakov published a treatise on instrumentation with examples from his own works, which remains widespread to this day. Check balance by listening, not at the piano.
Check, with arrangements of others' works, which interventions are necessary and which are mere habit, and give reasons for your decisions.