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rational (adj.)

late 14c., racional, "pertaining to or springing from reason;" mid-15c., of persons, "endowed with reason, having the power of reasoning," from Old French racionel and directly from Latin rationalis "of or belonging to reason, reasonable," from ratio (genitive rationis) "reckoning, calculation, reason" (see ratio).

In arithmetic, "expressible in finite terms," 1560s. Meaning "conformable to the precepts of practical reason" is from 1630s. Related: Rationally. It is from the same source as ratio and ration; the sense in rational is aligned with that in related reason (n.), which got deformed in French.

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Definitions of rational from WordNet
1
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
2
rational (n.)
an integer or a fraction;
Synonyms: rational number
From wordnet.princeton.edu