a number usually expressed in the form a/b.
a ratio of algebraic quantities similarly expressed.
Chemistry. (in a volatile mixture) a component whose range of boiling point temperatures allows it to be separated from other components by fractionation.
a part as distinct from the whole of anything; portion or section: The meeting started with a fraction of us present.
a very small part or segment of anything; minute portion: Only a fraction of the work was completed on time.
a very small amount; a little bit: It was only a fraction away from completion.
a piece broken off; fragment or bit.
the act of breaking.
Ecclesiastical. (in a Eucharistic service) the breaking of the Host.
verb (used with object), verb (used without object)
to divide or break into fractions, sections, factions, etc.: Dissension threatens to fraction the powerful union.

1350–1400; Middle English fraccioun < Late Latin frāctiōn- (stem of frāctiō) a breaking (in pieces), equivalent to Latin frāct(us) (past participle of frangere to break) + -iōn- -ion

subfraction, noun

3, 6. See part. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
fraction (ˈfrækʃən)
1.  maths
 a.  a ratio of two expressions or numbers other than zero
 b.  any rational number that is not an integer
2.  any part or subdivision: a substantial fraction of the nation
3.  a small piece; fragment
4.  chem a component of a mixture separated by a fractional process, such as fractional distillation
5.  Christianity the formal breaking of the bread in Communion
6.  the act of breaking
7.  (tr) to divide
[C14: from Late Latin fractiō a breaking into pieces, from Latin fractus broken, from frangere to break]

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., from L.L. fractionem (nom. fractio) "a breaking," especially into pieces, from root of L. frangere (pt. fregi) "to break," from PIE base *bhr(e)g- (cf. Skt. (giri)-bhraj "breaking-forth (out of the mountains);" Goth. brikan, O.E. brecan "to break;" Lith. brasketi "crash, crack;" O.Ir. braigim
"break" wind). Mathematical sense was the original one in English. Sense of "broken off piece, fragment," is from c.1600.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

fraction frac·tion (frāk'shən)

  1. An expression that indicates the quotient of two quantities.

  2. A chemical component separated by fractionation.

  3. A disconnected piece; a fragment.

  4. An aliquot portion or any portion.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
fraction  [%PREMIUM_LINK%]     (frāk'shən)  Pronunciation Key 
  1. A number that compares part of an object or a set with the whole, especially the quotient of two whole numbers written in the form a/b. The fraction 1/2 , which means 1 divided by 2, can represent such things as 10 pencils out of a box of 20, or 50 cents out of a dollar. See also decimal fraction, improper fraction, proper fraction.

  2. A chemical component separated by fractionation.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary

fraction definition

A mathematical expression representing the division of one whole number by another. Usually written as two numbers separated by a horizontal or diagonal line, fractions are also used to indicate a part of a whole number or a ratio between two numbers. Fractions may have a value of less than one, as with 1/2, or equal to one, as with 2/2, or more than one, as with 3/2. The top number of a fraction is the numerator and the bottom number is the denominator.

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
The unpaved section is a fraction of the total distance but takes a
  disproportionate amount of time.
The guess is then that fraction multiplied by the number of airplanes in, say,
  the current ten- or fifteen-minute window.
If they did, they couldn't afford to hire even a fraction of the number of
  adjuncts that they need.
It is a significant sum but a relatively small fraction of the world's total
  economic output.
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