Most of the terminology used by astronomers for spectral lines follows the definitions of the physicists. Let us recall briefly the meaning of some terms that are used in this book. For more details, the reader can consult any book on spectroscopy, like Atomic Spectra and Atomic Structure by Herzberg (1945) or Structure and Spectra of Atoms by Richards and Scott (1976).

An atom can have all its electrons, in which case it is said to be in the neutral state. If it has lost one electron it is said to be singly ionized and if it has lost n electrons it is said to be n times ionized. Degrees of ionization are indicated by Roman numerals – Nal is neutral sodium, Nail singly ionized sodium and so forth. If one wants to refer to any ionization stage of an atom then one speaks of species.

A spectral line is the result of the transition of an electron between two energy levels. Among the different energy levels available for an electron, some transitions are permitted by the selection rules. Among these one calls fundamental, resonance or ultimate lines those connecting a level to the lowest energy level. Usually these lines are the most intense ones.

Forbidden lines are formed by transitions between levels that are not allowed by the selection rules. Forbidden lines are indicated by the element enclosed in a square bracket – for example [Fell]. If the bracket appears only on the right-hand side – for example OIV] – this corresponds to a (forbidden) intersystem line.