Xenon gas
From Kerbal Space Program Wiki
Xenon gas is a resource that fuels ion engines.
Uses
There are only two xenon gas related parts available at the moment. No other uses aren't currently planned. The PB-X150 Xenon Container is a storage container and the PB-Ion Electric Propulsion System then uses this xenon to propel the craft. Currently the only one xenon container can't be placed radially. A larger radially xenon tank and a larger ion engine are included the KSPX addon.
