A plug is:

• a device which is designed to stop a liquid from flowing through a pipe.
• some type of connector that mates with a socket or jack, in particular used with electrical or electronic equipment or appliances
• in the entertainment industry, a type of promotion, usually by mentioning one's product during a show

In electronics, deciding which of two mating connectors is the plug and which the socket is sometimes arbitrary. In general:

• The male connector is the plug, the female the socket. This may pose a problem if both connectors have some male and female characteristics, for example the XLR3.
• The connector possessing the pins is the plug, the other the socket. This may pose a problem if both connectors have pins, for example the European 3-pin power connector.
• The connector normally mounted on a cable is the plug, and the connector mounted on the equipment is the socket. See DC plug for an example of this usage. This may pose a problem if the usage changes with time.