Arrangement (music)
An arrangement of a musical composition is a reworking of a piece of music so that it can be played by a different instrument or combination of instruments from the original.  For example, a song written for one voice with piano accompaniment might be arranged so that it can be sung in parts by a choir, or a piece for violin might be arranged so that it can be played on a clarinet instead.  The person who does the arrangement may be the composer themself, or it may be someone else.