Source : Webster's Revised Unabridged Dictionary (1913)

Liquefaction \Liq`ue*fac"tion\ (-f[a^]k"sh[u^]n), n. [L.
   liquefactio: cf. F. liqu['e]faction. See {Liquefy}.]
   1. The act or operation of making or becoming liquid;
      especially, the conversion of a solid into a liquid by the
      sole agency of heat.

   2. The state of being liquid.

   3. (Chem. Physics) The act, process, or method, of reducing a
      gas or vapor to a liquid by means of cold or pressure; as,
      the liquefaction of oxygen or hydrogen.

Source : WordNet®

liquefaction
     n : the conversion of a solid or a gas into a liquid