Wordnet 3.0
NOUN (1)
1.
a decrease in the density of something;
- Example: "a sound wave causes periodic rarefactions in its medium"
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]
WordNet (r) 3.0 (2006):
rarefaction
n 1: a decrease in the density of something; "a sound wave
causes periodic rarefactions in its medium"