Source : Webster's Revised Unabridged Dictionary (1913)

Rationale \Ra`tion*a"le\, n. [L. rationalis, neut. rationale.
   See {Rational}, a.]
   An explanation or exposition of the principles of some
   opinion, action, hypothesis, phenomenon, or like; also, the
   principles themselves.

Source : WordNet®

rationale
     n : (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: {principle}]