The monetary base is another name for reserve money (M0): the sum of notes and coin issued by the central bank and the balances that commercial banks hold in their accounts with it. The main entry explains how it is measured and what makes it change.
The term is used mostly in economics and in central bank statistics, where it is also called base money or high-powered money. It represents the liabilities of the central bank that serve as money, in contrast to the bank deposits created by commercial lending that make up most of the wider money supply. Usage varies slightly between countries: some central banks publish a narrow M0 that covers only currency in circulation, while their monetary base also includes bank reserves, split into required reserves and excess reserves.
The central bank controls the size of the base directly, expanding it when it buys assets or lends to banks and shrinking it when it does the reverse. The large asset purchases known as quantitative easing multiplied the monetary base in several economies after 2008 without a matching rise in broad money, which weakened the textbook idea of a fixed multiplier between the two.