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 thought
rational behavior
a process of rational inference
the triumph of the rational over the animal side of man
rational numbers
a rational analysis
rating
ratio
ratiocinate
ratiocination
ration
rational
rationale
rationalise
rationalist
rationality
rationalization
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