[rar.e.fac.tion] n [F or ML; F rarefaction, fr. ML rarefaction-, rarefactio, fr. L rarefacere to rarefy] (1603) 1: the action or process of rarefying
2: the quality or state of being rarefied
3: a state or region of minimum pressure in a medium traversed by compressional waves (as sound waves) -- rar.e.fac.tion.al adj