The Collaborative International Dictionary of English v.0.48:

Rationalize \Ra"tion*al*ize\ (r[a^]sh"[u^]n*al*[imac]z), v. t.
   1. To make rational; also, to convert to rationalism.
      [1913 Webster]

   2. To interpret in the manner of a rationalist.
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   3. To form a rational conception of.
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   4. (Alg.) To render rational; to free from radical signs or
      quantities.
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The Collaborative International Dictionary of English v.0.48:

Rationalize \Ra"tion*al*ize\, v. i.
   To use, and rely on, reason in forming a theory, belief,
   etc., especially in matters of religion: to accord with the
   principles of rationalism.
   [1913 Webster]

         Theodore . . . is justly considered the chief
         rationalizing doctor of antiquity.       --J. H.
                                                  Newman.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rationalize
    v 1: defend, explain, clear away, or make excuses for by
         reasoning; "rationalize the child's seemingly crazy
         behavior"; "he rationalized his lack of success" [syn:
         apologize, apologise, excuse, justify,
         rationalize, rationalise]
    2: weed out unwanted or unnecessary things; "We had to lose
       weight, so we cut the sugar from our diet" [syn: cut,
       prune, rationalize, rationalise]
    3: structure and run according to rational or scientific
       principles in order to achieve desired results; "We
       rationalized the factory's production and raised profits"
       [syn: rationalize, rationalise]
    4: think rationally; employ logic or reason; "When one wonders
       why one is doing certain things, one should rationalize"
       [syn: rationalize, rationalise]
    5: remove irrational quantities from; "This function can be
       rationalized" [syn: rationalize, rationalise]