The Great Depression was a period of about 10 years, 1929-1940. 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. The general term for decreasing national production, in modern economics, is a recession. 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 dominated by two recessions. The source for this information is the Federal Reserve Bank of St. Louis. This definition reflects the idea that unemployment is an excess supply of labor.