The Collaborative International Dictionary of English v.0.48:

Orchestration \Or`ches*tra"tion\, n. (Mus.)
   The arrangement of music for an orchestra; orchestral
   treatment of a composition; -- called also instrumentation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

orchestration
    n 1: an arrangement of a piece of music for performance by an
         orchestra or band
    2: the act of arranging a piece of music for an orchestra and
       assigning parts to the different musical instruments [syn:
       orchestration, instrumentation]
    3: an arrangement of events that attempts to achieve a maximum
       effect; "the skillful orchestration of his political
       campaign"

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "orchestration":
   Nachtmusik, absolute music, adaptation, air varie, aleatory,
   aleatory music, arrangement, chamber music, chamber orchestra,
   composition, descant, electronic music, etude, exercise,
   harmonization, incidental music, instrumental music,
   instrumentation, intonation, invention, modulation, nocturne, opus,
   phrasing, piece, preparation, production, program music,
   resolution, ricercar, score, setting, solution, sonata, sonatina,
   string orchestra, string quartet, study, suspension,
   theme and variations, tone painting, transcription, trio,
   variation, work