Arrangement

The term given to a particular version of a tune or song. Arrangements may specify types of Instruments, Key, number of instruments, Tempo and possibly even Rhythm and may add motifs and characteristics to the piece. Some arrangements add material to the original and some arrangments fuse two or more pieces together e.g. Simon and Garfunkel's Scarborough Fair / Canticle.

For the same song for example, different arrangements could be:

solo voice unaccompanied
multiple voices with Harmonies sung A Capella
solo voice with Acoustic Instruments
purely instrumental
with orchestra
with Brass Band
etc etc.