(1) A component you can add to a computer or other device to increase its capabilities. Add-ins can increase memory or add graphics or communications capabilities to a computer. They can come in the form of expansion boards, cartridges, or chips. The term add-in is often used instead of add-on for chips you add to a board that is already installed in a computer. In contrast, add-on almost always refers to an entire circuit board.
(2) A software program that extends the capabilities of larger programs. For example, there are many Excel add-ins designed to complement the basic functionality offered by Excel. In the Windows environment, add-ins are becoming increasingly common thanks to OLE 2.0.
(3) In Windows Home Server, add-in refers specifically to the supplemental programs that users can install to extend the capabilities of the Windows Home Server program. Most add-ins are third party or community developed.