Diffuse reflection is uniform reflection of light with no directional dependence for the viewer, e.g., a matte surface such as cardboard. Diffuse reflection originates from a combination of internal scattering of light, i.e. the light is absorbed and then re-emitted, and external scattering from the rough surface of the object. An illumination model must handle both direct diffuse reflection, i.e., light coming directly from a source to a surface and then reflected to the viewer, and indirect diffuse reflection (or diffuse interreflections), that is light coming from a source, being reflected to a surface, then reflected to another surface, etc, and finally to the viewer.
Some local illumination models, such as those that are usually used in many scan-line rendering and ray tracing systems, handle the diffuse interreflection contribution by simply assuming it has uniform intensity in all directions. This constant term is called the "ambient" contribution and it is considered a non-directional background light. Radiosity explicitly computes the diffuse interreflection term.