rarefaction

from WordNet (r) 3.0 (2006)

rarefaction
    n 1: a decrease in the density of something; "a sound wave
         causes periodic rarefactions in its medium"

from The Collaborative International Dictionary of English v.0.48

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]