Pronunciation: (ad'on", -ôn"),[key] —n. 1. a device or unit added to equipment or a construction: an add-on to a computer; a nice add-on to an old house. 2. an extra charge: Add-ons for taxes and tour guide fees boosted the price of the vacation to $2,500. 3. an additional item, as a rider or provision: This is just another legislative add-on. 4. anything added on: a hi-fi system that can be augmented with add-ons like extra speakers.
—adj. being installed or provided as an add-on: an add-on speaker system.