What is "absorption edge"

n. (context physics English) The boundary of a dark band in an x-ray spectrum.(R:MW3 1976: page=6)

An absorption edge, absorption discontinuity or absorption limit is a sharp discontinuity in the absorption spectrum of a substance. These discontinuities occur at wavelengths where the energy of an absorbed photon corresponds to an electronic transition or ionization potential. When the quantum energy of the incident radiation becomes smaller than the work required to eject an electron from one or other quantum states in the constituent absorbing atom the incident radiation ceases to be absorbed by that state. For example, incident radiation on an atom of a wavelength that has a corresponding energy just below the binding energy of the K shell electron in that atom cannot eject the K shell electron.