blackbody

A hypothetical body that cannot be excited to radiate by an external source of electromagnetic radiation of any frequency, direction, or state of polarization except in a negligibly small set of directions around that of the source radiation.
The traditional definition of a blackbody—as one that absorbs all the radiation incident on it—is inadequate unless to this definition is added the requirement that the body be large compared with the wavelength of the incident radiation. The concept of radiation incident on a body is from geometrical (or ray) optics, which is never strictly valid (because all bodies are finite) and may break down completely when the body is small compared with the wavelength. This was recognized by Planck, but by almost no one who followed him. Although no strict blackbody exists, some bodies are approximately black over a limited range of frequencies, directions, and polarization states of the exciting radiation.
See blackbody radiation, Planck's radiation law, emissivity.
Planck, M. 1959. The Theory of Heat Radiation. p. 2.

1. The process by which electromagnetic radiation is propagated through free space.