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. A device for recording, measuring, or controlling, especially such a device functioning as part of a control system. Music A device designed to enable a person to make musical sounds, as by blowing into it, striking it, depressing the keys on a keyboard, or plucking, strumming, or running a bow over strings. A means by which something is done; an agency:"The gods are just, and of our pleasant vices / Make instruments to plague us"(Shakespeare). One used by another to accomplish a purpose; a dupe. A legal document, especially one that represents a right of payment or conveys an interest, such as a check, promissory note, deed, or will. To provide or equip with instruments. Music To compose or arrange for performance.