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]