GOP is an acronym that stands for Grand Old Party in American politics and specifically refers to the Republican Party. GOP has also stood for Gallant Old Party and even Get Out and Push, both in reference to the Republican Party, but neither is its common usage.
The Grand Old Party label originally was applied to the Democratic Party by journalists as it predates the Republican Party by decades. Following the American Civil War, the term GOP appeared in the Congressional Record as the Gallant Old Party in reference to the Republicans because they had prevailed in the war. GOP gradually began to be used by journalists to refer to the Republicans, and the national party has appropriated the acronym to refer to itself.