Rendering equation
The integral that all photorealistic rendering tries to solve: the light leaving a point equals what it emits plus all incoming light weighted by the surface's reflectance.
Stated by James Kajiya in 1986 — David Immel and colleagues arrived at the same result independently that year. It says the light leaving a surface in a given direction is what the surface emits plus an integral over every incoming direction: incoming radiance times the surface's BRDF times a cosine term for the angle of incidence.
The equation is recursive — the light arriving has bounced off other surfaces solving the same integral — and has no closed-form solution outside trivial cases. Every rendering method is an approximation of it: ray tracing, radiosity and Monte Carlo global illumination each solve it to a different depth. Writing it down gave the field a single target to measure itself against.