Plug-in and Plug in

Plug-in noun - A printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities.

Plug in and plug-in are semantically related In some cases you can use "Plug in" instead a noun phrase "Plug-in".

Plug in verb - Plug into an outlet.

Plug-in and plug in are semantically related in link topic. Sometimes you can use "Plug-in" instead a noun "Plug in", if it concerns topics such as connection, part, element.