The document discusses gross domestic product (GDP) as a key measure of economic activity. It defines GDP as the total market value of all final goods and services produced within an economy in a given period. GDP equals the sum of consumption, investment, government spending, and net exports. While GDP is an important indicator of economic well-being, it does not capture all factors like leisure time, environmental quality, and non-market activities. The document also distinguishes nominal GDP based on current prices versus real GDP using constant prices adjusted for inflation.