What is "hohlraum"

n. (context physics English) A cavity whose walls are in thermodynamic equilibrium with the radiation emitted by the cavity

In radiation thermodynamics, a hohlraum (a non-specific German word for a "hollow area" or "cavity") is a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity. This idealized cavity can be approximated in practice by making a small perforation in the wall of a hollow container of any opaque material. The radiation escaping through such a perforation will be a good approximation to blackbody radiation at the temperature of the interior of the container.