[rash-uh-nl, rash-nl]
agreeable to reason; reasonable; sensible: a rational plan for economic development.
having or exercising reason, sound judgment, or good sense: a calm and rational negotiator.
being in or characterized by full possession of one's reason; sane; lucid: The patient appeared perfectly rational.
endowed with the faculty of reason: rational beings.
of, pertaining to, or constituting reasoning powers: the rational faculty.
proceeding or derived from reason or based on reasoning: a rational explanation.
capable of being expressed exactly by a ratio of two integers.
(of a function) capable of being expressed exactly by a ratio of two polynomials.
Classical Prosody. capable of measurement in terms of the metrical unit or mora.
Mathematics, rational number.

1350–1400; Middle English racional < Latin ratiōnālis, equivalent to ratiōn- (stem of ratiō) reason + -ālis -al1

rationally, adverb
rationalness, noun
antirational, adjective
antirationally, adverb
hyperrational, adjective
hyperrationally, adverb
nonrational, adjective
nonrationally, adverb
overrational, adjective
overrationally, adverb
prerational, adjective
quasi-rational, adjective
quasi-rationally, adverb
transrational, adjective
transrationally, adverb
ultrarational, adjective
ultrarationally, adverb
unrational, adjective
unrationally, adverb

rational, reasonable (see synonym study at reasonable).

2. intelligent, wise, judicious, sagacious, enlightened. 6. See reasonable.

2. stupid. 3. insane.
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
rational (ˈræʃənəl)
1.  using reason or logic in thinking out a problem
2.  in accordance with the principles of logic or reason; reasonable
3.  of sound mind; sane: the patient seemed quite rational
4.  endowed with the capacity to reason; capable of logical thought: man is a rational being
5.  maths expressible as a ratio of two integers or polynomials: a rational number; a rational function
6.  maths a rational number
[C14: from Latin ratiōnālis, from ratiōreason]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

late 14c., "endowed with reason," from L. rationalis "of or belonging to reason, reasonable," from ratio (gen. rationis) "reckoning, calculation, reason" (see ratio). Rationalist "physician whose treatment is based on reason" is from 1620s; applied to a philosophical doctrine
1640s. Rationalize is first recorded 1803, "to explain, to make reasonable;" in the psychological sense of "to give an explanation that conceals true motives" it dates from 1922.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

rational ra·tion·al (rāsh'ə-nəl)

  1. Having or exercising the ability to reason.

  2. Influenced by reasoning rather than by emotion.

  3. Of sound mind; sane.

  4. Based on scientific knowledge or theory rather than practical observation.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Example sentences
Rationally, there is no justification for this behavior.
But they also all behave rationally in response to the economic incentives
  those distortions create.
Actually, because those are lifetime appointments it's the only category where
  filibusters can be rationally defended.
Thus, advertisers are behaving rationally when they buy online advertising,
  which is more efficient and more easily measured.
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