Cryogenic propellants are rocket propellants that are stored at very low temperatures. Cooling some common room-temperature gases allow them to be used as high-performance rocket propellants. Typically, cryogenic propellants are stored in a liquid state at their boiling point. Examples of Cryogenic propellants are LOX (liquid oxygen) and LH2 (liquid hydrogen).
Meyer, Michael L., David J. Chato, David W. Plachta, Gregory A. Zimmerli, Stephen J. Barsi, Neil T. Van Dresar, and Jeffrey P. Moder. "Mastering cryogenic propellants." Journal of Aerospace Engineering 26, no. 2 (2012): 343-351.