Cygwin/X is an implementation of the X Window System that runs under Microsoft Windows. It is part of the Cygwin project, and is installed using Cygwin's standard setup system. As with the rest of Cygwin, Cygwin/X is free software, licensed under the GNU General Public License (GPL).
Cygwin/X can run rootless. This allows X applications to draw their windows directly onto the normal Windows desktop, rather than into an enclosing root window.