A Whig is either a member of the 19th century American political party, or a a member of a British political party that originated in the 17th century.
In the United States, the Whigs were active from 1833 to 1854. Whigs supported the supremacy of Congress over the President. The party was founded as an organized protest against the "tyranny" of President Andrew Jackson and his economic policies. Though American Whigs were in agreement about national, economic development through industry and modernization, they were split on the issue of slavery. This divide eventually led to the party's crumble, with most joining either the Republican or Know-Nothing parties.
In Great Britain, the Whig party was active from 1678 to 1859. Great Britain's Whig party rivaled the Tory party. The Whigs valued the abolition of slavery, tolerance for nonconformist Protestants, free trade, and a constitutional monarchy where Parliament would have more power than the Monarch. They were not a strongly organized party, and most Whigs found stronger leadership and better organization as part of the Liberal Party.
Today, loosely organized and rather minority political parties calling themselves "Whigs" can be found in both England and the United States.