The alien world in Half-Life. Seems to consist of floating islands linked by teleporters. The sky is very strange and there is less gravity. You meet Gonarch and the Nihilanth here. The world of Xen seems to rely on biological technology, such as living lights and elevators.

Some theories on Xen:
1.) The G-Man refers to it as a border world... Implying that there is something on the other side of the border?
2.) Could it be a gas giant? The lack of real land and the weird skies would suggest this.