What is the server-focused product family of Microsoft Windows called?

The server-focused product family of Microsoft Windows is called Windows Server.

Windows Server is Microsoft's family of operating systems designed for server computers rather than ordinary personal computers. It provides infrastructure features such as file and print services, user and permissions management, networking, web hosting through Internet Information Services, virtualization through Hyper-V, and directory services through Active Directory.

The family began with Windows NT 3.1 Advanced Server, released in 1993. Microsoft used the name Windows NT Server for several generations before adopting the shorter Windows Server branding with Windows Server 2003. The naming can cause confusion because “Windows” by itself usually refers to Microsoft's desktop operating system, while Windows Server identifies the server-oriented branch.

Windows Datacenter and Windows Enterprise are not separate product families in the same sense. They have been edition or licensing labels used for particular server or client releases. Windows Workstation is also not the canonical name of Microsoft's server product line. Windows Server editions may offer a desktop interface, but their licensing, tools, roles, and intended workloads distinguish them from consumer Windows.