emissivityn.

1880 Encycl. Brit. XI. 577/2 We define thermal emissivity as the quantity of heat per unit of time.

1880 Encycl. Brit. XI. 577/2 The first thoroughly trustworthy experiments giving emissivities in absolute measure.

1884 P. G. Tait Light 248 We now define the emissivity of a body at a given temperature, for a particular radiation, as the ratio of its emission of that radiation to the emission of the same radiation by a black body at the same temperature.

1891 Proc. Royal Soc. 50 166 (heading) The thermal emissivity of thin wires in air.

1902 Encycl. Brit. XXVII. 189/2 Fourier defined another constant expressing the rate of loss of heat at a bounding surface per degree of difference of temperature between the surface of the body and its surroundings. This he called External Conductivity, but the term Emissivity is more convenient.

1958 H. J. Gray Dict. Physics 174/1 Emissivity of a surface is the ratio of its emissive power to that of a black body for a given wavelength and at the same temperature. The term is also used for the amount of heat emitted per second by unit area of the surface maintained at a temperature of one degree above its surroundings, but a better name..is heat transfer coefficient.