A wildcat strike is a strike action undertaken by unionised workers without union leadership's authorisation, support, or approval; this is sometimes termed an unofficial industrial action. The legality of wildcat strikes varies between countries and over time.

Strike action, also called labor strike, labour strike and industrial action in British English, or simply strike, is a work stoppage caused by the mass refusal of employees to work. A strike usually takes place in response to employee grievances. Strikes became common during the Industrial Revolution, when mass labor became important in factories and mines. As striking became a more common practice, governments were often pushed to act. When government intervention occurred, it was rarely neutral or amicable. Early strikes were often deemed unlawful conspiracies or anti-competitive cartel action and many were subject to massive legal repression by state police, federal military power, and federal courts. Many Western nations legalized striking under certain conditions in the late 19th and early 20th centuries.