rationale

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}]
    

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]
    

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