1. In general, any financial security such as a bond, stock, check, etc. Money market securities (such as a Treasury Bill, U.S. government bonds, or commercial paper) and capital market securities (such as a mortgage, Certificate of Deposit, or long-term bonds) are also referred to as instruments. 2. A document containing some legal right or obligation. Examples include notes, agreements, and contracts. 3. Any tool that can be used in order to implement economic policy. For example, the government may use interest rates as an instrument for implementing monetary policy, and taxes as an instrument for implementing fiscal policy.