Installed base

Installed base is a measure of the number of units of a product or service that are actually in use, especially software or an Internet or computing platform, as opposed to market share, which only reflects sales over a particular period. Although the install base number is often created using the number of units that have been sold within a particular period, it is not necessarily restricted to just systems, as it can also be products in general. For products which are in use on some machines for many years, the installed base count will be higher than sales over a given period. Some people see it as a more reliable indicator of a platform's usage rate.
Installed base is not the same as the total number of units sold at any given moment in time, since some of those units will typically be out of use because they have broken, gone missing, or been made obsolete and/or replaced by newer versions.