Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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:
         {instrumentation}]
     3: an arrangement of events that attempts to achieve a maximum
        effect; "the skillful orchestration of his political
        campaign"