Microfacet model
Treating a surface as a sea of microscopic perfect mirrors whose orientations follow a statistical distribution — the basis of all modern PBR highlights.
Rather than measure every bump, the model assumes a surface is made of countless tiny facets. A matte surface has facets pointing every which way and scatters light; a glossy one has facets lying nearly parallel and produces a tight highlight. Roughness is simply the width of that distribution.
Cook–Torrance brought the idea into graphics in 1982, building on Torrance–Sparrow's 1967 optics. A microfacet BRDF has three factors: D, how the facet normals are distributed (GGX, from Trowbridge–Reitz 1975, became standard after Walter 2007); F, the Fresnel reflectance; and G, how facets shadow and mask one another. Disney and Unreal turned it into the real-time norm around 2012–2013.