The Collaborative International Dictionary of English v.0.48:

Instrumental \In`stru*men"tal\, a. [Cf. F. instrumental.]
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   1. Acting as an instrument; serving as a means; contributing
      to promote; conductive; helpful; serviceable; as, he was
      instrumental in conducting the business.
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            The head is not more native to the heart,
            The hand more instrumental to the mouth. --Shak.
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   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.
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            Sweet voices mix'd with instrumental sounds.
                                                  --Dryden.
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   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.
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   Instrumental errors, those errors in instrumental
      measurements, etc., which arise, exclusively from lack of
      mathematical accuracy in an instrument.
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