I. (ˈ)i, ə, (ˈ)ir, (ˈ)iə+ adjective
Etymology: Middle English irrationall, from Latin irrationalis, from in- in- (I) + rationalis rational — more at rational
: not rational: as
a.
(1) : not endowed with reason : lacking powers of reasoning or understanding
the lower animals are commonly described as irrational
(2) : lacking usual or normal mental clarity or coherence
was irrational for several days after the accident
b. : not governed by or according to reason
irrational … is a neutral term meaning either what is outside the scope of reason or what has not yet been tested by reason — Times Literary Supplement
c. Greek & Latin prosody
(1) of a syllable : having a quantity other than that required by the meter
(2) of a foot : containing such a syllable
(3) of a meter : containing such feet
d. of a number : real but not expressible as the quotient of two integers
π and √3 are irrational numbers
II. “+ noun
( -s )
1. : an irrational being : a being not acting according to reason
2. : an irrational quantity or number : surd