Arrangement
In popular music the arrangement is the selection of instrumentation, the indicated or composed material to be read or improvised by instrumentalists, and the chart or score of this material if one exists. Every piece of music has an arrangment, which may or may not be done by a proffessional arranger. An unscored chart is called a head chart (it is in the head of the arranger(s)).An arrangement often refers specifically to the adaptation of a previously arranged piece of music for another musical application than for which it was originally meant. This includes arrangements for a different instrument, or different instruments (for example an arrangement for piano or flute, or a duet, based on a symphonic piece; or arrangement of instrumental accompaniment for vocal music) and for another musical style (for example adaptation of a classical piece into a more modern style of music).
In classical music arrangements are often orchestrations, which is the scoring or re-scoring for full orchestra.
Further reading