in·stru·ment n
1: a means or implement by which something is achieved, performed, or furthered
an instrument of crime
2: a document (as a deed, will, bond, note, certificate of deposit, insurance policy, warrant, or writ) evidencing rights or duties esp. of one party to another under the law
no person is liable on an instrument unless his signature appears thereon — Uniform Commercial Code
an indictment is a form of charging instrument; specif: negotiable instrument
Merriam-Webster’s Dictionary of Law. Merriam-Webster. 1996.