An Etymological Dictionary of Astronomy and Astrophysics
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1) A measure of the → strength of a → spectral line. The equivalent width is the width of a → rectangle centered on a spectral line that, on a plot of → intensity against → wavelength, has the same → area as the line.
2) The width-integrated → optical depth of a → planetary ring. For rings with very small optical depths, the equivalent width is very nearly equal to the equivalent depth (Ellis et al., 2007, Planetary Ring Systems, Springer).

→ equivalent; → width.