Windows 2000 is the 5th mainline entry in the Microsoft Windows franchise, following Windows 95. It was first released on December 15th, 1999 worldwide, though a Japanese release wouldn't come until December 24th, 1999. It was the first mainline Windows version to use the revamped NT kernel, though it was originally used within the spin-off series "Windows NT". Windows 2000 was also the first mainline Windows version to be muti-platform on launch, coming out for the NEC PC-9821, Dell OptiPlex series GC, IBM PC (1990s reboot), and IBM Thinkpad.
Windows 2000 mainly included features from Windows 95 alongside its spin-off/direct sequel "Windows 98", including Plug-and-Play, DirectX, Internet Explorer, and a few other improvements.
Windows 2000 is often considered to be the last truly mainline entry in the Microsoft Windows franchise, since the line would split in two with the release of Windows XP and Windows Server 2003 in 2001 and 2003 respectively.