Thermal switch. Probably goes open at 185C.
Being picky but it's not a thermal switch, because that implies it's resettable. A thermal fuse is a one shot device: once it's blown, it'll need replacing. Thermal switches are also often used for thermal protection, but they're normally combined with a thermal fuse, just in case the contacts weld closed.
Thermal fuses are normally used for fire protection. Quite often the device will be irreparable before the thermal fuse blows, but not always, so it might be worth replacing. Try temporally replacing it with a wire and if the device works, including the temperature controller (it's very common for a temperature controller to fail with the heater stuck on, causing the thermal fuse to blow), seek a replacement. Obviously
don't permanently replace it with a wire, because that would result in a huge fire risk.