'Rarefaction' definitions:

Definition of 'rarefaction'

(from WordNet)
noun
A decrease in the density of something; "a sound wave causes periodic rarefactions in its medium"

Definition of 'Rarefaction'

From: GCIDE
  • Rarefaction \Rar`e*fac"tion\ (r[a^]r`[-e]*f[a^]k"sh[u^]n), 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. [1913 Webster]