An Etymological Dictionary of Astronomy and Astrophysics
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The absorption and prompt re-emission of photons of a particular wavelength by an atom. In this process, a photon of exactly the right wavelength (i.e. energy) excites an electron in the atom from one energy level to another. The electron then drops back down to its original energy level more or less immediately, emitting a photon of almost identical energy to the one that was absorbed in the first place, but in some random direction. Resonant scattering applies only to line radiation, unlike other forms of scattering which are of continuous radiation (Oxford Dictionary of Astronomy, 2 ed.).

→ resonant; → scattering.