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The Lick indices are a set of spectral line indices, defined methods to measure a set of specific stellar spectral lines, devised for use in stellar population synthesis (SPS). The measurements have known relations to the age and constituents of stars.
The Lick indices are the result of an effort using observation data from Lick Observatory to relate characteristics of stars with their spectral features by creating a library (catalog) of stars of known characteristics (e.g., age, metallicity, spectral class) and the values of their Lick indices. With this library, spectra of populations such as the stars of a distant (possibly unresolvable) galaxies and stellar clusters can be compared to mixes of known types of stars to deduce characteristics of the entire galaxy or cluster, e.g., age and metallicity, clues to their history.
Each Lick index corresponds to a spectral line, consisting of a comparison of the strength of a narrow spectral band centered on the line to that of the surrounding continuum, the comparison expressed as a magnitude (log of their ratio). The definitions of the indices indicate the wavelength-intervals used to characterize the strength of the index's line and the continuum surrounding it. A correlation between (functions of) these indices and independently-determined metallicity values for the library of stars yields a formula to approximate a population's metallicity values based on its Lick index values.