RAREFAC'TION, noun [See Rarefy.] The act or process of expanding or distending bodies, by separating the parts and rendering the bodies more rare or porous, by which operation they appear under a larger bulk, or require more room, without an accession of new matter; opposed to condensation; as the rarefaction of air.
Source: Webster's 1828 American Dictionary ↗ · public-domain · returned verbatim, no AI