Definition of: irrational

(i·rash′ən·əl) adjective
1. Not possessed of or not exercising reasoning powers.
2. Math. a Pertaining to an irrational number. b Denoting an algebraic expression containing variables that are irreducible, as , . Compare RATIONAL.
3. Contrary to reason; absurd.
4. In Greek and Latin prosody, not keeping the proper ratio between thesis and arsis, as a long syllable used in place of a short one. See synonyms under ABSURD, INSANE. [<L irrationalis]
—ir·ra′tion·al·ly adverb