Apple software that adds server functionality to any Mac machine running a current version of macOS.  Called "OS X Server" when it debuted with OS X Version 10.7 (Lion), macOS Server adds centralized management of desktop and mobile devices, file sharing, backup, calendars, mail, wikis and Xcode deployment.  A Web server lets users publish their own website, and Xsan provides a shared storage area network (SAN).
Introduced in 1999, Mac OS X Server was pre-installed on various Mac models that were sold as dedicated servers.  As of OS X 10.7, the software became available from the Mac App Store for any Mac.  See 
macOS and 
Xserve.