1. Not rational; void of reason or understanding; as, brutes are irrational animals.
2. Not according to reason; absurd; foolish. It seemed utterly irrational any longer to maintain it. (i. Taylor)
3. (Science: mathematics) Not capable of being exactly expressed by an integral number, or by a vulgar fraction; surd; said especially of roots. See surd.
Synonym: absurd, foolish, preposterous, unreasonable, senseless. See absurd.
Origin: L. Irrationalis: cf. F. Irrationnel. See In- not, and rational.