Much of the analysis of astronomy involves identifying spectral lines, ideally comparing them with lines produced in a laboratory and calculated through quantum mechanics, and short of that, calculated heuristically through patterns discerned in experiment. To write about and discuss lines, designators for the lines are used, which vary widely in form, but there are general conventions:
The chemical symbol for an element or molecule is often used, either to indicate all the lines it produces, or in context, to refer to a particular line, or as part of a designator that indicates a particular line or a subset. Ionized atoms or molecules produce different lines, and the superscript + or - might be used (e.g., H+) but in astronomy, a Roman-numeral convention is commonly used: I means neutral, II means singly (positively) ionized, i.e., missing an electron, III means doubly ionized (missing two), etc., for example, H I and H II for hydrogen. Surrounding square brackets (e.g., [CII]) indicates a forbidden line, a line detectable in astronomical observation whose creation requires so much extremely diffuse material that laboratories cannot reproduce it. Such a designator may refer to all its lines, or in context, a particular line or a subset. Different isotopes can also produce differing lines, e.g., the 21-cm line versus the analogous line of deuterium, and the indication of a specific isotope (a preceding superscript, e.g., 2H) might be used.
A specific line may be indicated by following one of the above with a wavelength, often indicated by its measure in angstroms or nanometers, e.g., "Fe5270". Sometimes a λ is included, and sometimes all that is indicated is the wavelength, e.g., "Ca I λ 4226" or just "λ 4226". In the radio-end of the spectrum, the case for many molecular lines, there is more chance that a line will be identified by frequency. An apparent line can be caused by two related transitions that produce photons of nearly identical wavelength, which may be observed as two lines, or may show as one due to line broadening or insufficient spectral resolution. A designator may ignore this if not of interest, or may indicate the individual lines by some means. Two lines from different elements or molecule species might be so close together that they blend when both are present in a source: the symbol BL (for "blend") is sometimes used to indicate such a blended (or ambiguous) line.
A specific line may also be indicated by two numbers corresponding to the electron shell transition, or other kinds of excitation quantum steps of molecules, e.g., CO(2-1). If of interest, the line may be indicated by specifications of the electron orbitals before and after the transition, in particular, in cases where there are lines of very nearly identical wavelength, per above. For X-rays electron shells are commonly indicated by letters, K for 1, L for 2, etc., and lines are often indicated by the lower-energy shell, e.g., iron K-line.
A subscript may appear after the symbol: I can't tell for sure what this means: sometimes it looks to be part of the formula for a molecule, and sometimes, if after an element's chemical symbol it apparently merely distinguishes two or more well-known lines. A very large subscript (e.g., 5238) undoubtedly is a wavelength or frequency.
Certain lines have designations of very long standing, e.g., Fraunhofer lines (a letter code, sometimes subscripted), and series lines, such as H-alpha, with an indication of the series (H for Balmer series, L or Ly for Lyman series), such as Lyman alpha (Ly-α), typically using Greek letters, α, β, etc., to indicate the line within the series. Other lines acquire common names merely for being of high interest, such as the 21-cm line.
The term line index is sometimes used, perhaps merely indicating the designator, but often indicating a number which is a measure of the strength of the line, in which case designators as described above may be used for to indicate that number, as measured from some source. The measurement may be specific to a certain kind of observation, e.g., a specific photometric system or spectroscopy, like a color index but more specific. In general, the goal is to characterize the difference spectral energy at the peak of the line with that of the continuum emission on either side. A typical means of specifying the ratio is as a magnitude. For exactness, a writer may specify the specific method by which this was done.
Among the uses of line identification and strengths is the determination of spectral types of stars, studies in metallicity, and studies of groups of stars (stellar cluster or galaxies) regarding their demographics.