business jargon The number units of a particular product or service that are currently in use, especially software or an Internet or computing platform. (Also referred to as "install(ed) user base" or simply "install(ed) base.") There have been fears that the computer giant's existing user base may decline after their newest operating system is implemented. The video game developer's insistence on backwards compatibility in their newest console shows an earnest effort to keep their user base happy.