An object's emissivity is the degree to which the total power of its thermal emission matches that of a black body of the same temperature, expressed as a fraction or percentage. The fraction is always less than one. Absorptance (aka absorptivity) is the inverse, the ratio of received EMR that an object absorbs. Emissivity of an object typically depends upon the temperature of the object, and its spectral emissivity (fraction of black-body radiation power at a particular wavelength) generally varies by wavelength, and is often cited or graphed indicating these specifics. For example, an instrument's emissivity may be cited as measured at wavelength λ, when the instrument's temperature is t.
A telescope or instrument's emissivity is of interest, especially in far infrared and microwave equipment in creating highly sensitive instruments: the instruments that are often cooled to cryogenic temperatures to improve their sensitivity. A low emissivity "leverages" the benefits of the low temperature.