Original: Windows 2000 is a continuation of the Microsoft Windows NT family of operating systems, replacing Windows NT 4.0. Originally called Windows NT 5.0, then Windows NT 2000, Microsoft changed the name to Windows 2000 on 27 October 1998.
Simplified: Windows 2000 (also known as Win2K, W2K or Windows NT 5.0) is an operating system that was designed to work with computers that have either single or multiple processors.