total market value of the goods and services produced by a nation's economy during a specific period of time. It includes all final goods and services—that is, those that are produced by the economic resources located in that nation regardless of their ownership and that are not resold in any form. GDP differs from gross national product (GNP), which is includes all final goods and services produced by resources owned by that nation's residents, whether located in the nation or elsewhere. In 1991 the United States substituted GDP for GNP as the main measure of economic output.