1. A tool or implement used to do or facilitate work, especially a small precision tool used by a professional: sterilized the scalpel and other surgical instruments.
2. A device for recording, measuring, or controlling, especially such a device functioning as part of a control system.
3. Music A device for playing or producing music: a keyboard instrument.
4. A means by which something is done; an agency: "The gods are just, and of our pleasant vices / Make instruments to plague us" (Shakespeare).
5. One used by another to accomplish a purpose; a dupe.
6. A legal document, especially one that represents a right of payment or conveys an interest, such as a check, promissory note, deed, or will.http://www.thefreedictionary.com/instrument