Radiance is a term in radiometry. It is defined as:
L = d 2 Φ d A d Ω c o s θ {\displaystyle L={\frac {d^{2}\Phi }{dAd\Omega cos\theta }}}
Where Φ {\displaystyle \Phi } is radiant flux, A {\displaystyle A} is an area, and Ω {\displaystyle \Omega } is a solid angle.
Typical units are [ W m 2 s r ] {\displaystyle [{\frac {W}{m^{2}sr}}]} where "sr" means steradians.