v. t. [ imp. & p. p. Rarefied p. pr. & vb. n. Rarefying ] [ F. raréfier; L. rarus rare + -ficare (in comp.) to make; cf. L. rarefacere. See -fy. ] To make rare, thin, porous, or less dense; to expand or enlarge without adding any new portion of matter to; -- opposed to condense. [ 1913 Webster ]

v. i. To become less dense; to become thin and porous. “Earth rarefies to dew.” Dryden. [ 1913 Webster ]