1. Plug can be a rather generic term for any small flat or flat unit that fits into a hole or gap in a wall of flats or scenery. Plugs are also employed as a way to back a section of scrim or projection screen in an otherwise solid wall. The plug is removed for the duration of the effect, then replaced afterwards.
A related term from the film/TV industry would be "Wild Wall".
2. Electrical or Signal. Noun. A line/cord-mounted wiring device having male pins or contacts for temporary connection intended to be inserted into a receptacle, jack, or connector. In some circles, a plug is always male, and a connector is always female, thus no such thing as a "female plug" or "male connector". Electrical contractors refer to a plug, such as a Hubbell 5266, as a "cord cap".
2a. Verb. To insert or remove the above.