From AMS Glossary
(Often called diffraction zone.) May be used for any region that would not be illuminated by a given source of electromagnetic (or acoustic) radiation if it propagated strictly according to ray optics in a homogeneous (on the scale of the wavelength) medium.
Some radiation, however, does penetrate shadow zones because of scattering by the propagating medium or by obstacles within it. Because of the widespread misconception that diffraction is fundamentally different from scattering, the term diffraction zone is also used, especially by radio engineers, who might say that the earth diffracts radio waves into this zone (and probably would ignore scattering by the atmosphere). Yet all radiation that penetrates a shadow zone does so because of scattering by the atmosphere and by solid and liquid bodies (including the earth) within the atmosphere or at its boundary.