An object's emissivity is the degree to which the total power of its thermal emission matches that of an ideal 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 generally 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 some specific wavelength when the instrument is at some specific temperature.
A telescope or instrument's emissivity is of interest, especially for far infrared and microwave equipment in developing high sensitivity, i.e., those instruments that are often cooled to cryogenic temperatures to improve their sensitivity. A low general emissivity can "leverage" (increase) the benefits of the low temperature.