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]