Definition and background:
The selection and adaptation of a composition or parts of a composition to instruments for which it was not originally designed or for some other use for which it was not at first written. an adaptation of a given composition into a form other than as originally composed.
For more information on the music term "Arrangement" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary