An Etymological Dictionary of Astronomy and Astrophysics

The state of being rarefied, less dense.
An instantaneous reduction in density of a gas resulting from passage a sound wave;
opposite of → compression.

Etymology (EN): M.E. rarefien, from M.Fr. rarefier, from L. rarefacere “make rare,” from rarus “loose, wide apart, thin, infrequent.”

Etymology (PE): Verbal noun from âlar present stem of âlaridan→ rarefy + -š, a suffix.