Of the three main political parties in the USA (Democratic, Republican, Independent), the GOP stands for "Grand Old Party." It is an old nickname for the Republican Party.
The first Republican president was Abraham Lincoln. After the Civil War, the upstart Republicans were perceived as the party that won the war. The Republicans who were firmly fixed into the Federal Government were ironically dubbed the "Gallant Old Party," which soon became known as the "Grand Old Party," which was then shortened to the "GOP." The Republicans are conservative, favor reducing the size and role of the government, and they support the rights of the individual. This ideology is most recognized as that of Republican President Ronald Regan.