1Of water: somewhat cooler than normal body temperature; (Medicine) designating a bath or other treatment in which patients are exposed to water that is cooler than body temperature (now rare).
Of a particle, especially a neutron or electron: having less energy than would be possessed by a particle in thermal equilibrium with its environment. Of energy, etc.: below that associated with thermal equilibrium.
Mid 19th century (in an earlier sense). From sub- + thermal.