Cel shading
Also said as Toon shading, Toon shader
Shading in a few flat steps of colour instead of a smooth gradient, often with an ink outline, so a 3D model looks like a hand-painted animation cel.
It gives a game a strong, lasting look. It is also unforgiving: every small lump in a surface becomes a hard blotch of shadow, so the surface directions have to be tended by hand.
Say it
“Cel shade the characters with three light steps and a hard edge between them, plus a dark outline two pixels thick. Hand-edit the face normals so the nose shadow stays clean.”
Where you’ve seen it
Jet Set Radio (2000) made the look famous. Guilty Gear Xrd took it to a point where its 3D fighters pass for 2D; Junya Motomura's GDC 2015 talk explains that the slightest difference in a surface normal can end up as a huge blotch.
Not to be confused with outline
Outlines are a separate pass around a shape. Cel shading is the stepped lighting inside it. Each can appear without the other.