Black Body

A black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name “black body”  refers to its ability to absorb all colours of light. A black body also emits black-body radiation. In contrast, a white body is one with a "rough surface that reflects all incident rays completely and uniformly in all directions. (source = https://en.wikipedia.org/wiki/Black_body) 

How we use it as lighting designers? 

Lighting designers use the black body as a reference because its emission represents the spectrum closest to the sun. When selecting an LED for example, we always pay attention to its spectral distribution and how closely it matches that of the black body.