Source : Webster's Revised Unabridged Dictionary (1913)

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

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

   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.

            Sweet voices mix'd with instrumental sounds.
                                                  --Dryden.

   3. (Gram.) Applied to a case expressing means or agency; as,
      the instrumental case. This is found in Sanskrit 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.

   {Instrumental errors}, those errors in instrumental
      measurements, etc., which arise, exclusively from want of
      mathematical accuracy in an instrument.

Source : WordNet®

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}]