"In the case where a cryogenic propellant is kept at a temperature below its boiling point but above its freezing point, the propellant is considered sub-cooled. This is typically done to increase the density of the propellant and therefore increase the amount of mass that can be pumped assuming a constant volume flow rate." (Vaniman, Worlund, and Winstead 1969)
Vaniman, J. L., A. L. Worlund, and T. W. Winstead. "Slush and subcooled propellants for lunar and interplanetary missions." In Advances in Cryogenic Engineering, pp. 20-29. Springer, Boston, MA, 1969.