From AMS Glossary
A radiometric term for the rate at which radiant energy in a set of directions confined to a unit solid angle around a particular direction is transferred across unit area of a surface (real or imaginary) projected onto this direction.
Unlike irradiance, radiance is a property solely of a radiation field, not of the orientation of the surface. The SI units of radiance are W m-2 sr-1. In general, radiance depends on time, position, and direction as well as frequency (monochromatic or spectral radiance) or range of frequencies. Irradiance for any surface is the integral of radiance over a hemisphere of directions above or below that surface. The photometric equivalent of radiance is luminance, obtained by integrating spectral radiance weighted by luminous efficiency over the visible spectrum.