Definition of "add-on"

add-on

noun
plural add-ons
1. (sometimes attributive) Something which can be appended to something else.
  Quotations
  Payments made before an add-on sale must be allocated entirely to the previous contract or contracts.
  1969, Edward D. Giacomini, California Retail Installment Sales, page 172
2. (computer hardware, software) A software extension or hardware peripheral that provides additional functions or customization for a core application or system.
  Quotations
  Microdrives that can hold several gigabytes (see Figure 2.20) have been available as add-ons for portable devices.
  2010, Kenneth Baldauf, Ralph Stair, Succeeding with Technology, page 77
3. (poker) Additional chips that can be purchased at specific times in tournament play.