In chemistry, an unstable system which has a greater concentration of a material in solution than would exist at equilibrium is said to be supersaturated.
In meteorology, supersaturation of an air mass with respect to H2O vapour is of special interest. It is the saturation ratio minus one, or the percent supersaturation is the percent relative humidity minus 100.
Source: PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2217 (https://doi.org/10.1351/pac199062112167)
See also: Orange Book, 2nd ed., p. 84 [Terms] [Book]