An American white supremacist, right-wing terrorist, and hate group whose primary targets are African Americans, Jews, Latinos, Asian Americans, Catholics, and Native Americans as well as immigrants, leftists, homosexuals, Muslims, abortion providers and atheists.

The Klan has existed in three distinct eras. The first Klan used terrorism—both physical assault and murder—against politically active Black people and their allies in the Southern United States in the late 1860s. The third Klan used murders and bombings from the late 1940s to the early 1960s to achieve its aims. All three movements have called for the "purification" of American society, and are all considered far-right extremist organizations.

In each era, membership was secret and estimates of the total were highly exaggerated by both friends and enemies.

The first Klan was established in the wake of the American Civil War and was a defining organization of the Reconstruction era. Organized in numerous chapters across the Southern United States, federal law enforcement suppressed it around 1871. It sought to overthrow the Republican state governments in the South, especially by using voter intimidation and targeted violence against African-American leaders. Each chapter was autonomous and highly secretive about membership and plans. Members made their own, often colorful, costumes: robes, masks and conical hats, designed to be terrifying and to hide their identities.