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Published online by Cambridge University Press: 06 September 2019
Before starting to survey recent observations related to stellar chromospheres, an operational definition of a chromosphere is needed; such definition must satisfy two requirements: (1) it must be bound to a set of observables which we agree indicate the presence of a chromosphere; (2) it must be reasonable in terms of the physical effects which we say characterize a chromosphere. Indeed one does not want to be a priori confined to call chromospheric indicators only those spectral features which, in the Sun, have been attributed to the chromospheric regions of formation of the spectrum, and which, by analogy, can be said to be a sign of a chromosphere in stars similar enough to the Sun.