Per-pixel lighting

From Tech Artists Wiki

Per-pixel lighting usually refers to the use of shaders, specifically the Pixel Shader, and the evaluation of lighting per-pixel, instead of interpolated from vertex to vertex (as in Per-Vertex Lighting, or Gouraud shading). This yields much more believable and less polygonal lighting. More importantly, though, it allows the use of per-pixel effects and transformations- normal mapping is one such effect that is common enough to become ubiquitous with per-pixel lighting.

For more info, see Shaders and Portal:Shaders.