M1

M1 is a narrow definition of the money supply that includes currency, traveler's checks, and checking accounts in banks.

Definition

According to Principles of Economics by Timothy Taylor (3rd edition),

M1. A narrow definition of the money supply that includes currency, traveler's checks, and checking accounts in banks.

According to Macroeconomics by Mankiw (7th edition),

M1. Various measures of the stock of money, where larger numbers signify a broader definition of money.