[ ih-rash-uh-nl ]
/ ɪˈræʃ ə nl /



Mathematics. irrational number.

Nearby words

  1. irradiance,
  2. irradiant,
  3. irradiate,
  4. irradiation,
  5. irradicable,
  6. irrational number,
  7. irrationalism,
  8. irrationality,
  9. irrationalize,
  10. irrawaddy

Origin of irrational

First recorded in 1425–75; late Middle English word from Latin word irratiōnālis. See ir-2, rational

Related forms Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for irrational

British Dictionary definitions for irrational


/ (ɪˈræʃənəl) /


inconsistent with reason or logic; illogical; absurd
incapable of reasoning
  1. not rational
  2. (as noun)an irrational
prosody (in Greek or Latin verse)
  1. of or relating to a metrical irregularity, usually the occurrence of a long syllable instead of a short one
  2. denoting a metrical foot where such an irregularity occurs
Derived Formsirrationally, adverbirrationalness, noun

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for irrational



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.

Online Etymology Dictionary, © 2010 Douglas Harper

Medicine definitions for irrational


[ ĭ-răshə-nəl ]


Not rational; marked by a lack of accord with reason or sound judgment.

The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.