Radiant flux per unit area per unit solid angle per unit wavelength interval, i.e. W/m2/sr/nm.
In intuitive terms, it is the quantity we humans perceive as brightness. When viewing an illuminated object or light source we perceive it as having the same brightness regardless of our distance from it, even though the absolute flux we receive from the object depends on distance. Similarly a camera?an imaging system not unlike our eyes?is sensitive to the radiance at each point in an image. Radiance may also be measured with a non-imaging radiance collector.
Radiance is a function of viewing angle, and other radiometric quantities such as irradiance can be calculated by integrating the radiance over a range of angles.