The phrase self-absorption refers to the absorption by a substance of electromagnetic radiation (EMR) emitted by the same type of substance. Absorption mechanisms are the converse of emission mechanisms, such that any wavelength emitted by a substance can be absorbed by that substance. For example, if a particular atom produces a photon through the energy release of an electron moving from a higher electron shell (atomic excitation) to a lower shell, then the photon can be absorbed by an atom of the same element, pushing its electron on the opposite course, like the emission mechanism working backwards. This is a key mechanism in radiative transfer. Thermal radiation is essentially EMR that has undergone so much self-absorption that the resulting spectrum statistically converges into a particular pattern (black-body spectrum). Spectrums other than black-body are modified by any lesser amounts of self-absorption, modifications sometimes recognizable in spectral features. Some particular cases of interest in astronomy are synchrotron self-absorption, HI self-absorption (HISA, self-absorption of the 21-cm line), and [CII] self-absorption. It is also a contributor to line broadening.