The Collaborative International Dictionary of English v.0.48:

Rarefy \Rar"e*fy\ (r[a^]r"[-e]*f[imac]; 277), v. t. [imp. & p.
   p. Rarefied (r[a^]r"[-e]*f[imac]d); p. pr. & vb. n.
   Rarefying (r[a^]r"[-e]*f[imac]`[i^]ng).] [F. rar['e]fier;
   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]

The Collaborative International Dictionary of English v.0.48:

Rarefy \Rar"e*fy\, v. i.
   To become less dense; to become thin and porous. "Earth
   rarefies to dew." --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rarefy
    v 1: lessen the density or solidity of; "The bones are rarefied"
    2: make more subtle or refined [syn: rarefy, sublimate,
       subtilize]
    3: weaken the consistency of (a chemical substance) [syn:
       rarefy, attenuate]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "rarefy":
   add to, adulterate, aggrandize, amplify, attenuate, augment,
   baptize, bloat, blow up, broaden, build, build up, bulk, bulk out,
   crescendo, cut, dematerialize, develop, dilate, dilute, disembody,
   distend, emacerate, emaciate, enervate, enlarge, etherealize,
   expand, extend, hike, hike up, huff, increase, inflate, irrigate,
   magnify, puff, puff up, pump, pump up, raise, reduce, sap,
   spiritualize, stretch, subtilize, sufflate, swell, thin, thin away,
   thin down, thin out, up, water, water down, weaken, widen