Webster's English Dictionary

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rarefaction

rar.efac.tion \.rar-*-'fak-sh*n, .rer-\ \-shn*l, -sh*n-*l\ \-'fak-tiv\ n [F 
   or ML; F rare`faction, fr. ML rarefaction-, rarefactio, f]r. L rarefactus, 
   pp. of rarefacere to rarefy 1: the act or process of rarefying  2: the 
   quality or state of being rarefied  - rar.efac.tion.al aj