The Collaborative International Dictionary of English v.0.48:

Instrumental \In`stru*men"tal\, a. [Cf. F. instrumental.]
   [1913 Webster]
   1. Acting as an instrument; serving as a means; contributing
      to promote; conductive; helpful; serviceable; as, he was
      instrumental in conducting the business.
      [1913 Webster]

            The head is not more native to the heart,
            The hand more instrumental to the mouth. --Shak.
      [1913 Webster]

   2. (Mus.) Pertaining to, made by, or prepared for, an
      instrument, esp. a musical instrument; as, instrumental
      music, distinguished from vocal music. "He defended the
      use of instrumental music in public worship." --Macaulay.
      [1913 Webster]

            Sweet voices mix'd with instrumental sounds.
                                                  --Dryden.
      [1913 Webster]

   3. (Gram.) Applied to a case expressing means or agency; as,
      the instrumental case. This is found in Sanskrit and
      Russian as a separate case, but in Greek it was merged
      into the dative, and in Latin into the ablative. In Old
      English it was a separate case, but has disappeared,
      leaving only a few anomalous forms.
      [1913 Webster]

   Instrumental errors, those errors in instrumental
      measurements, etc., which arise, exclusively from lack of
      mathematical accuracy in an instrument.
      [1913 Webster]

WordNet (r) 3.0 (2006):

instrumental
    adj 1: relating to or designed for or performed on musical
           instruments; "instrumental compositions"; "an
           instrumental ensemble" [ant: vocal]
    2: serving or acting as a means or aid; "instrumental in solving
       the crime" [syn: implemental, instrumental,
       subservient]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "instrumental":
   accessory, adjuvant, advantageous, agential, agentival, agentive,
   ancillary, assistant, assisting, auxiliary, beneficial, catalytic,
   concert, conducive, contributory, dramatico-musical, employable,
   facilitating, favoring, forwarding, fostering, handy, helpful,
   helping, important, intermediary, jazz, jazzy, mediating,
   mediatorial, ministerial, ministering, ministrant, nurtural,
   nutricial, of service, orchestral, promoting, rock, serviceable,
   serving, significant, subservient, subsidiary, supporting,
   supportive, swing, symphonic, syncopated, useful, utilitarian,
   valuable