A recession is defined as a period of two or more successive quarters of decreasing production.
Production is measured by a number of variables.
Real Gross Domestic Product is one important measure. We will focus mainly on it. The Great Depression was a period of about 10 years, 1929-1940, dominated by two recessions. Here is a diagram that shows something of the decline in production in the Great Depression. The green line in Figure 1 is Gross National Product in dollars of 1929 purchasing power.