A tourniquet is a device that controls blood flow to an extremity by applying circumferential pressure. It is used medically to create a bloodless surgical field or stop traumatic bleeding. The earliest known usage was by Romans in 199 BC during amputations. Modern tourniquets are used widely in orthopedic and plastic surgery. Complications can include post-tourniquet syndrome if applied for too long or at insufficient pressure, especially in patients with vascular issues. Proper application and monitoring can prevent complications.