Refers to a product designed to complement another product. For example, there are numerous add-on boards available that you can plug into a personal computer to give it additional capabilities. Another term for add-on board is expansion board.
Add-on products are also available for software applications. For example, there are add-on report generation programs that attach to popular database products such as dBASE, giving them additional report-generation and graphics capabilities.
The terms add-on and add-in are often, but not always, used synonymously. The term add-in can refer to individual chips you can insert into boards that are already installed in your computer. Add-on, on the other hand, almost always refers to an entire circuit board, cartridge, or program.