, of a source is the integral of the spectral radiance, , over the source solid angle:

The unit is named after pioneering US radio astronomer Karl Guthe Jansky and is defined as{\cdot}\mathrm{Hz^{-1}} (SI)| 2 = (CGS).}}Since the jansky is obtained by integrating over the whole source solid angle, it is most simply used to describe point sources; for example, the Third Cambridge Catalogue of Radio Sources