Source : Webster's Revised Unabridged Dictionary (1913)

Rarefaction \Rar`e*fac"tion\, n. [Cf. F. rar['e]faction. See
   {Rarefy}.]
   The act or process of rarefying; the state of being rarefied;
   -- opposed to condensation; as, the rarefaction of air.

Source : WordNet®

rarefaction
     n : a decrease in the density of something; "a sound wave causes
         periodic rarefactions in its medium"