A plug on a piece of electrical equipment is a small plastic object with two or three metal pins which fit into the holes of an electric socket and connects the equipment to the electricity supply.
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插座
A plug is an electric socket.
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（浴缸或水槽的）塞子
A plug is a thick, circular piece of rubber or plastic that you use to block the hole in a bath or sink when it is filled with water.
If someone plugs a commercial product, especially a book or a film, they praise it in order to encourage people to buy it or see it because they have an interest in it doing well.