from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Designed to be plugged in to an electric circuit: a plug-in game.
- n. Computer Science See add-in.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Designed to be plugged into an electrical power outlet or circuit.
- n. A computer program module or device that interacts with another to add a specific function, or to support a specific file format or device.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. a printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities
From the verb to plug in. (Wiktionary)