Incident luminous energy specified by its spectral distribution.

something that can serve as a source of light

A light defined by its spectral power distribution. An illuminant may or may not be physically realizable as a source. Several standard illuminants have been defined by the CIE for use in colorimetric computations. See source.

Mathematical description of the relative spectral power distribution of a real or imaginary light source, that is, the relative energy emitted by a source at each wavelength in its emission spectrum.