Each pixel of a texture during the time the texture is being processed by the RT3D engine. After the engine performs calculations to project the texture onto polygons, the texture pixels are transformed into texels. Then the engine renders the scene, and at that point it transforms those texels into screen pixels.
The distiction between texels and pixels is important in defining how the engine transforms textures. First they're texture pixels, then they're texels, then they're finally screen pixels.