This phrase was used of an appliance that connected to an electrical outlet; now it's an add-on for a browser
When a homeowner decides to fix an outlet themselves, it's important that
... [DOC] [TLE] Plug - definition of plug by The Free Dictionary [PAR] An object, such as a cork or a wad of cloth, used to fill a hole tightly; ... for 
insertion in a fixed socket, used to connect an appliance to a power supply.