An Etymological Dictionary of Astronomy and Astrophysics

1. General: The state or quality of being opaque.
   

2. A measure of the absorption of photons on their way from the stellar center to the surface. Opacity depends upon the frequency of the radiation, the density, the chemical composition, and the thermodynamic state of the gas. For a given density, the hotter the gas the lower the opacity, since the gas absorbs less readily, as described by → Kramers’ law.
   Conversely, the cooler the gas the higher the opacity. See also → stellar pulsation, → kappa mechanism, → valve mechanism, → partial ionization zone.

Etymology (EN): From Fr. opacité, from L. opacitatem (nom. opacitas) “shade, shadiness,” from opacus “shaded, dark, opaque.”

Etymology (PE): Kederi, from keder “opaque,” from Ar. kader + -i suffix forming nouns from adjectives.