MatCap
A pre-rendered sphere image sampled by the surface normal in view space — fast stylized shading that ignores the scene's lights entirely.
The idea (material capture, also "lit sphere") comes from Sloan et al. in 2001: photograph or paint a lit sphere, then use a vertex's screen-projected normal as a lookup into that image. The whole material — lighting, reflection, color — is baked into one texture, so a complete shading model collapses into a single cheap texture read.
That is why MatCap first showed up in sculpting tools like ZBrush and Blender, where you want to read form without rigging a light setup, and it still appears on the web (Sketchfab) and on mobile. The drawback is just as obvious: the lighting is burned in and does not follow the object as it moves through a scene.