instrument

1. General: Device that communicates, denotes, detects, indicates, measures, observes, records, or signals a quantity or phenomenon, or controls or manipulates another device.

2. Law: Formally executed document that evidences a legally enforceable agreement between two or more parties, and expresses a contractual duty, obligation, or right. Practically all documents used in borrowing, lending, investing, and sale and purchase on credit, are legal instruments.