Instrumental

a.
  1. Acting as an instrument; serving as a means; contributing to promote; conductive; helpful; serviceable; as, he was instrumental in conducting the business.
  2. (Mus.) Pertaining to, made by, or prepared for, an instrument, esp. a musical instrument; as, instrumental music, distinguished from vocal music.
  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.
    4. Instrumental errors ,
    those errors in instrumental measurements, etc., which arise, exclusively from lack of mathematical accuracy in an instrument.