The phrase self-absorption refers to the absorption by a substance of electromagnetic radiation (EMR) emitted by that substance. Absorption mechanisms are the reverse of emission mechanisms. 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 might be absorbed by an atom of the same element, pushing its electron on the opposite course, like the emission working backwards. Every type of emission has such an absorption converse. This is a factor in radiative transfer, and for optically thin clouds, affects the spectrum of the emerging EMR, and can sometimes be recognized by 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.