in·stru·ment (nstr-mnt)

A means by which something is done; an agency.
One used by another to accomplish a purpose; a dupe.
An implement used to facilitate work.
A device for recording, measuring, or controlling, especially such a device functioning as part of a control system.
Music A device for playing or producing music: a keyboard instrument.
A legal document, such as a deed, will, mortgage, or insurance policy.