1Meteorology. More fully snow water equivalent. The amount of precipitation represented by a quantity of snow, sleet, or ice.
2Physics. The amount of water, measured in terms of mass, which requires the same amount of heat as a given body in order to raise its temperature by the same amount. Now rare.
The water equivalent of a particular body is calculated by multiplying the mass of the body by its specific heat..
Origin
Mid 19th century (in an earlier sense). From water + equivalent.