Micropolygon
A polygon diced down to roughly half a pixel, so that shading and displacement can happen per surface-covering fragment without visible edges.
The concept comes from Pixar's REYES architecture (Cook, Carpenter, Catmull, 1987). Instead of shading large triangles, surfaces are diced into a grid of micropolygons, each covering less than a pixel. Displacement, motion blur and shading can then be computed uniformly and finely, which gave film rendering its smooth surfaces.
The snag is efficiency: fixed-function rasterization hardware wastes nearly all its work when a triangle is smaller than a pixel, because it always processes at least a 2×2 quad. That is why Nanite in Unreal Engine 5 built its own software rasterizer specifically for micropolygons. The same insight drives research into efficient sub-pixel geometry on the GPU.