add-on

noun

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.

adjective

5. being installed or provided as an add-on: an add-on speaker system.

noun

1. a feature that can be added to a standard model or package to give increased benefits

n.

“additional component,” 1941, from add (v.) + on.