Add-in

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.

A mini program which runs in conjunction with a web browser or other application that enhances the functionality of that progran.In order for the add-in to run,the main application must be running as well.