The Collaborative International Dictionary of English v.0.48:

Rationale \Ra`tion*a"le\ (r[a^]sh`[u^]*n[a^]l" or
   r[a^]sh`[u^]n*[=a]"l[-e]), n. [L. rationalis, neut.
   rationale. See Rational, a.]
   An explanation or exposition of the principles of some
   opinion, action, hypothesis, phenomenon, or the like; also,
   the principles themselves.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rationale
    n 1: (law) an explanation of the fundamental reasons (especially
         an explanation of the working of some device in terms of
         laws of nature); "the rationale for capital punishment";
         "the principles of internal-combustion engines" [syn:
         rationale, principle]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "rationale":
   account, allegorization, clarification, cracking, decipherment,
   decoding, demonstration, demythologization, editing, elucidation,
   emendation, enlightenment, euhemerism, excuse, exegesis,
   exemplification, explanation, explication, exposition, expounding,
   grounds, illumination, illustration, justification, light, logic,
   philosophy, pretense, pretext, principle, rational ground,
   rationalization, reason, reason for, reason why, reasoning,
   simplification, solution, stated cause, the big idea, the idea,
   the whatfor, the wherefore, the why, theory, underlying reason,
   unlocking