Xen (pronounced "Zen") is the origin of the alien species that appear in the science fiction video game Half-Life (Valve Software, 1998); the expansion packs Half-Life: Opposing Force (Valve Software and Gearbox Software, 1999); Half-Life: Blue Shift (Valve/Gearbox, 2001); and Half-Life: Decay (Valve/Gearbox, 2001) - as well as many of those that appear in Half-Life 2 (Valve Software, 2004). Xen is sometimes referred to as the border world.