A term attributed to liquid rocket propellants that only become liquid after being cooled and/or pressurized. These propellants are more powerful (in terms of specific impulse) than storable propellants, but since they must be kept cold and pressurized, storage and handling nightmares often result.
One of the most powerful common cyrogenic propellants is liquid hydrogen/liquid oxygen, which has a specific impulse of 450 seconds in a vacuum.