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. Pertaining to, made by, or prepared for, an instrument, especially. A musical instrument; as, instrumental music, distinguished from vocal music. He defended the use of instrumental music in public worship. Sweet voices mix'd with instrumental sounds. (Dryden)
3. 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.