Wiki says (in regards to a musical arrangement):
In music, an arrangement is a musical reconceptualization of a previously composed work.[1] It may differ from the original work by means of reharmonization, melodic paraphrasing, orchestration, or development of the formal structure. Arranging differs from orchestration in that the latter process is limited to the assignment of notes to instruments for performance by an orchestra, concert band, or other musical ensemble. Arranging "involves adding compositional techniques, such as new thematic material for introductions, transitions, or modulations, and endings.... Arranging is the art of giving an existing melody musical variety".
Dictionary definition of the word as a whole:
noun
1.
the action, process, or result of arranging or being arranged.
"the arrangement of the furniture in the room"
synonyms: positioning, disposition, order, presentation, display; More
a thing that has been arranged in a neat or attractive way.
"flower arrangements"
2.
plans or preparations for a future event.
"all the arrangements for the wedding were made"
an agreement with someone.
"the travel agents have an arrangement with the hotel"
synonyms: agreement, deal, understanding, bargain, settlement, pact, modus vivendi
"we had an arrangement"
3.
MUSIC
a composition adapted for performance with different instruments or voices than those originally specified.
"Mozart's symphonies in arrangements for cello and piano"
synonyms: adaptation, orchestration, instrumentation
"an arrangement of Beethoven's symphonies"
4.
archaic
a settlement of a dispute or claim.