Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. Mathematics An expression that indicates the quotient of two quantities, such as 1/3 .
 n. A disconnected piece; a fragment.
 n. A small part; a bit: moved a fraction of a step.
 n. A chemical component separated by fractionation.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. A part of a whole, especially a comparatively small part.
 n. A ratio of two numbers, the numerator and the denominator, usually written one above the other and separated by a horizontal bar.
 n. A component of a mixture, separated by fractionation.
 n. In a eucharistic service, the breaking of the host.
 n. A small amount.
 v. To divide or break into fractions.
from the GNU version of the Collaborative International Dictionary of English
 n.
 n. The act of breaking, or state of being broken, especially by violence.
 n. A portion; a fragment.
 n. One or more aliquot parts of a unit or whole number; an expression for a definite portion of a unit or magnitude.
 transitive v. To separate by means of, or to subject to, fractional distillation or crystallization; to fractionate;  frequently used with out.
from The Century Dictionary and Cyclopedia
 n. The act of breaking, or the state of being broken, especially by violence; a breaking or fracture.
 n. Specifically (ecclesiastical), the liturgical act of breaking or dividing the eucharistic bread, or host.
 n. A fragment; a separated portion; a disconnected part.
 n. In mathematics: In arithmetic, a part, or a number of aliquot parts, of unity.
 n. In algebra, a ration of algebraic quantities analogous to the arithmetical vulgar fraction, and similarly expressed.
 n. In mathematics: In geometry, any multiple of any submultiple of a magnitude.
 n. In chem., one of the parts into which a substance is separated in the process of fractional distillation. See fractionation.
 Same as fractionate.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. a small part or item forming a piece of a whole
 n. the quotient of two rational numbers
 v. perform a division
 n. a component of a mixture that has been separated by a fractional process
Etymologies
Examples

Loop if (y0 = y1) break if (fraction > = 0) x0+ = stepx, fraction  = dy y0+ = stepy, fraction+ = dx

Loop if (x0 = x1) break if (fraction > y0+ = stepy, fraction  = dx x0+ = stepx, fraction+ = dy

And in Sweden today the guard labor fraction is less than half that of the United States.
LowTrust Society?, Arnold Kling  EconLog  Library of Economics and Liberty

Because the piles are typically large enough (in the case of most publicly traded corporations), merely bleeding off a small fraction of a small fraction is enough to make a man wealthy beyond the capacity of most people to imagine.

The glazing contains a form of uraniumoxide, and a certain fraction of the uranium238 nuclei will absorb the moderated neutrons, and thereby transform to uranium239.

Except that a certain fraction of the CA voting public has by now gotten wise to the thing, and there are now a lot of people (me included) who vote “no” on ALL CA initiatives unless provided with a compelling reason to do otherwise.

Apparently there was a great many lessons learned, a certain fraction (estimated at between 1 and 3 quarters of the total) of decent material produced, and apparently a lot of the problems related to people not knowing or agreeing on … what the goals and success criteria for the project were.
hughstimson.org » Blog Archive » The Finnicky Wisdom of Crowds

Only a small fraction of the nuclear collisions in the Sun succeed in overcoming this repulsion and causing fusion; this fraction is very sensitive to the temperature.

A certain fraction of the electrons will penetrate the barrier by tunnelling and we may obtain a weak tunnel current through the barrier.

Measured by Gross National Product (GNP), at the end of World War II, we were more than forty per cent of the whole; today the fraction is less than a quarter.
zc0000 commented on the word fraction
fraction
in fraction
March 1, 2010
oroboros commented on the word fraction
fRAcTIOn
August 4, 2008
reesetee commented on the word fraction
"How many adults are living in your household? Please respond with a whole number."  actual wording on a survey question
January 11, 2008