The "M2 Money Supply", also referred to as "M2 Money Stock", is a measure for the amount of currency in circulation. M2 includes M1 (physical cash and checkable deposits) as well as "less liquid money", such as saving bank accounts.
The chart above plots the yearly change in M2 and the Inflation Rate, which is defined as the yearly change in the Consumer Price Index (CPI). When inflation is high, prices for goods and services rise and thus the purchasing power per unit of currency decreases.
Historically, M2 has grown along with the economy (see in the chart below). However, it has also grown in war times along with Federal Debt to GDP. In most recent history, M2 growth surpassed 10 percent in recessions, during which an expansionary monetary policy was deployed by the central bank, including large scale asset purchases. According to Bannister and Forward (2002, page 28), Money supply growth and inflation are inexorably linked.
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