1. adjective
a) Acting as an instrument; serving as a means; contributing to promote; conductive; helpful; serviceable; essential or central.
He was instrumental in conducting the business.
b) Pertaining to, made by, or prepared for, an instrument, especially a musical instrument; as, instrumental music, distinguished from vocal music.
See Also: instrumentation, instrumentality, instrumentive
2. noun
a) The instrumental case.
Wikipedia foundation.