These terms are monetary aggregates used by economists to classify the money supply based on liquidity. M0 represents "base money," including physical currency in circulation and reserves held by banks. M1 is the most liquid measure, consisting of M0 plus demand deposits (like checking accounts). M2 builds on M1 by including savings accounts, money market securities, and small-denomination time deposits. M3, often called "broad money," incorporates M2 along with larger, less liquid assets such as institutional money market funds, long-term repurchase agreements, and large time deposits. M4 is the broadest measure, sometimes used to include even wider arrays of liquid assets, varying by the specific central bank's definition.