Curve of growth
From AMS Glossary
curve of growth
For an isolated, gaseous absorption band, the relation between integrated absorption by the gas and its amount.
That is, if
where κ is the (volumetric) absorption coefficient of the gas, x is the path length through it, and the range of integration over frequency ν is sufficient to include essentially all absorption within the band, then W as a function of x is the curve of growth. This term, which Goody describes as "curious," is a relic from astronomical spectroscopy.
Goody, R. M. 1964. Atmospheric Radiation. Vol. 1, . p. 125.