Noun
etendue (uncountable)
(optics) A conserved property of the light in an optical system which characterizes how "spread out" the light is in terms of angle and area: it is the product of its cross-sectional area (normal to the direction of propagation) and the solid angle it subtends.
Synonyms: AΩ product, acceptance, geometric extent, light-collecting power, light-gathering power, light grasp, optical extent, throughput