[1] : The Collaborative International Dictionary of English v.0.48

rarefaction \rar`efac"tion\ ra^r`-efa^k"shu^n, n.
   cf. f. rar'efaction. see rarefy.
   the act or process of rarefying; the state of being rarefied;
   -- opposed to condensation; as, the rarefaction of air.
   1913 webster
see also:
rarefy 

[2] : WordNet (r) 2.0

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

Rarefaction >> R Words
Rarefaction, definition of term: Rarefaction
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