An Etymological Dictionary of Astronomy and Astrophysics

A collisionally excited spectral line formed in high density condition well above the → critical density. At such densities the → excitation temperature is at (or very near) the → kinetic temperature
of the gas. At low densities, below the critical density, the excitation temperature will be only slightly above the radiation temperature and the emission line will be practically invisible.

See also: Thermalized, p.p. of → thermalize; → line.