late 14c., "pertaining to reason;" mid-15c., "endowed with reason," from Old French racionel and directly from Latin rationalis "of or belonging to reason, reasonable," from ratio (genitive rationis) "reckoning, calculation, reason" (see ratio).
rational ra·tion·al (rāsh'ə-nəl)
Having or exercising the ability to reason.
Influenced by reasoning rather than by emotion.
Of sound mind; sane.
Based on scientific knowledge or theory rather than practical observation.