The thing on top of a pressure cooker is commonly referred to as the "pressure cooker lid" or simply the "lid." It is an essential component of a pressure cooker that seals the cooking vessel, allowing pressure to build up inside and raise the boiling point of the liquid, resulting in faster cooking times. The lid typically has a handle for easy lifting and a locking mechanism to secure it in place during pressurized cooking. Additionally, pressure cooker lids often include a pressure release valve or pressure regulator to control and release excess pressure when necessary for safety.