radiance, L

Radiant power, $P$ , leaving or passing through a small transparent element of surface in a given direction from the source about the solid angle $Ω$ , divided by the solid angle and by the orthogonally projected area of the element in a plane normal to the given beam direction, $d S ⊥ = d S cos ⁡ θ$

Notes:

Mathematical definition:
$L = d 2 P d Ω d S ⊥ = d 2 P d Ω d S cos ⁡ θ$
for a divergent beam propagating in an elementary cone of the solid angle $Ω$ containing the direction $θ$ . SI unit is $W m −2 sr −1$ .
For a parallel beam it is the radiant power, $P$ , of all wavelengths leaving or passing through a small element of surface in a given direction from the source divided by the orthogonally projected area of the element in a plane normal to the given direction of the beam, $θ$ . Mathematical definition in this case: $d P / ( d S cos ⁡ θ )$ If the radiant power is constant over the surface area considered, $P / S cos ⁡ θ$ . SI unit is $W m −2$ .
Equivalent to $L = ∫ λ L λ ⁢ d λ$ , where $L λ$ is the spectral radiance at wavelength $λ$ .

Source:

PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 408