The term line broadening (often just broadening) refers to the mechanisms that create the shape of an absorption line (e.g., from a star), that has breadth from mechanisms that affect a range of wavelengths. The mechanisms include Doppler broadening due to the Doppler effects of the movement of hot absorbing atoms, natural broadening from the Heisenberg uncertainty principle, which assures no atom has a fixed velocity, and collisional broadening (pressure broadening) from distortion due to absorption by an atom whose absorption voltages are distorted by a recent collision. The spectral line's width is quantified by an equivalent width, meaning the width of a theoretical maximum-depth rectangular-shaped line with the same area as the given spectral line. An alternate measure is full width at half maximum, a measure of the width "half way down" the line. Line depth is the depth of the line as a percentage of the continuum of the spectrum on either side of the line.
A curve of growth is a plot of a model relating a line's equivalent width to the number of absorbing atoms in the column through which the light passes to create an absorption line.
Rotational broadening is broadening due to the rotation of a body, i.e., the part rotating away from the observer is redshifted and the part rotating toward is blue shifted, both according to the radial velocity of that part of the body. Analysis reveals information about the rotation of planets, stars, galaxies and clouds.