rational (adj.)

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).

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Definitions of rational from WordNet
rational (adj.)
consistent with or based on or using reason;
rational thought
rational behavior
a process of rational inference
rational (adj.)
of or associated with or requiring the use of the mind;
the triumph of the rational over the animal side of man
Synonyms: intellectual / noetic
rational (adj.)
capable of being expressed as a quotient of integers;
rational numbers
rational (adj.)
having its source in or being guided by the intellect (as distinguished from experience or emotion);
a rational analysis
rational (n.)
an integer or a fraction;
Synonyms: rational number