A tourniquet was a device, often improvised, used to cut off blood flow to an injured limb.
In 2372, B'Elanna Torres compared Automated Unit 3947's power depletion to a bleeding artery, saying she didn't have a tourniquet. (VOY: "Prototype")
In 2373, Miles O'Brien told a badly injured Enrique Muniz that he would "tighten this bandage like a tourniquet" if he didn't stop complaining about the first aid the engineer was providing to him. (DS9: "The Ship")