[n] 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"
\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.
WordNet Dictionary