irrational

[ih-rash-uh-nl] /ɪˈræʃ ə nl/

adjective

without the faculty of reason; deprived of reason.

without or deprived of normal mental clarity or sound judgment.

not in accordance with reason; utterly illogical:

irrational arguments.

not endowed with the faculty of reason:

irrational animals.

Mathematics.

(of a number) not capable of being expressed exactly as a ratio of two integers.
(of a function) not capable of being expressed exactly as a ratio of two polynomials.

Algebra. (of an equation) having an unknown under a radical sign or, alternately, with a fractional exponent.

Greek and Latin Prosody.

of or pertaining to a substitution in the normal metrical pattern, especially a long syllable for a short one.
noting a foot or meter containing such a substitution.

noun

Mathematics, irrational number.

Origin

1425-75; late Middle English < Latin irratiōnālis. See ir-², rational

Related forms

irrationally, adverb

irrationalness, noun

nonirrational, adjective, noun

nonirrationally, adverb

nonirrationalness, noun

Synonyms

3. unreasonable, ridiculous; insensate.

Examples from the web for irrational

Though seemingly irrational, this pattern of faculty politics is familiar to anyone who has worked in a university.
It seems irrational to feel the trees closing behind you, as if the forest is cutting you off from the present.
Bright's courage was almost as irrational as that of the rebels themselves.
It's time to rein in the irrational exuberance of those who raise political money.
Unfortunately, almost any argument in favor of nuclear weapons today is likely to be seen as irrational or worse.
Please do your research before making such sweeping, totally irrational statements.
But those of us with memories that extend to the previous millennium do detect a bit of irrational exuberance.
While certainly tragic, it was far from irrational: it turns out the behavioral economics were stacked against them.
Suits are a bizarre intrusion from an irrational world.
One of the trickiest problems in dealing with an uncertain future is people's seemingly irrational response to it.

British Dictionary definitions for irrational

irrational

/ɪˈræʃənəl/

adjective

inconsistent with reason or logic; illogical; absurd

incapable of reasoning

(maths)

not rational
(as noun): an irrational

(prosody, in Greek or Latin verse)

of or relating to a metrical irregularity, usually the occurrence of a long syllable instead of a short one
denoting a metrical foot where such an irregularity occurs

Derived Forms

irrationally, adverb
irrationalness, noun

Word Origin and History for irrational

adj.

late 15c., "not endowed with reason" (of beats, etc.); earlier (of quantities) "inexpressible in ordinary numbers" (late 14c.); from Latin irrationalis "without reason," from assimilated form of in- "not, opposite of" (see in- (1)) + rationalis "reason" (see rational). Meaning "illogical, absurd" is attested from 1640s. Related: Irrationally.

irrational in Medicine

irrational ir·ra·tion·al (ĭ-rāsh'ə-nəl)
adj.
Not rational; marked by a lack of accord with reason or sound judgment.