a device or unit added to equipment or a construction: an add-on to a computer; a nice add-on to an old house.
an extra charge: Add-ons for taxes and tour guide fees boosted the price of the vacation to $2,500.
an additional item, as a rider or provision: This is just another legislative add-on.
anything added on: a hi-fi system that can be augmented with add-ons like extra speakers.
being installed or provided as an add-on: an add-on speaker system.
1945–50; noun and adj. use of verb phrase add on