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.
      [1913 Webster]

   3. To form a rational conception of.
      [1913 Webster]

   4. (Alg.) To render rational; to free from radical signs or
      quantities.
      [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

117 Moby Thesaurus words for "rationalize":
   account, account for, adjust, allegorize, analyze, apply reason,
   argue insincerely, arrange, calculate, cast, chart, clarify, clear,
   clear up, codify, concert, contrive, coordinate, crack,
   cry sour grapes, cut out, decipher, deduce, demonstrate,
   demythologize, design, destigmatize, devise, distort,
   do justice to, elucidate, enlighten, euhemerize, exculpate, excuse,
   exemplify, explain, explain away, explicate, exposit, expound,
   figure, fix, forecast, frame, generalize, give reason for,
   give the meaning, harmonize, hypothesize, illuminate, illustrate,
   infer, intellectualize, intend, justify, lay plans, logicalize,
   logicize, make a projection, make arrangements, make clear,
   make plain, methodize, misapply, normalize, organize, paralogize,
   pervert, philosophize, plan, plan ahead, popularize, prearrange,
   program, project, provide a rationale, purge, reason, reason ill,
   reason out, regularize, regulate, rehabilitate, reinstate, restore,
   routinize, schedule, schematize, set up, settle, shape,
   shed light upon, show, show how, show the way, simplify, solve,
   spell out, standardize, syllogize, synchronize, synthesize,
   systematize, theorize, think through, throw light upon, tune,
   tune up, unfold, unlock, unravel, use reason, vindicate, warrant,
   work out, work up