add-in

Add-in is a term used, especially by Microsoft, for a software utility or other program that can be added to a primary program. The Microsoft Style Guide says that Microsoft Bookshelf is an add-in for Word and that Analysis Toolpak is an add-in for Microsoft Excel. According to the Style Guide, add-in should not be confused with add-on , a term for a hardware expansion unit. (However, some add-on manufacturers do call them "add-ins.")

A similar term is plug-in , a term originated by Netscape for application programs that can be activated within a Netscape Web browser window.