the act or process of expanding.
the state or quality of being expanded.
the amount or degree of expanding.
an expanded, dilated, or enlarged portion or form of a thing: The present article is an expansion of one he wrote last year.
anything spread out; expanse.
the development at length of an expression indicated in a contracted form, as a 2 + 2 ab + b 2 for the expression (a + b ) 2 .
any mathematical series that converges to a function for specified values in the domain of the function, as 1 + x + x 2 + … for 1/(1 − x ) when x < 1.
Machinery. that part of the operation of an engine in which the volume of the working medium increases and its pressure decreases.
an increase in economic and industrial activity (opposed to contraction ).

1605–15; < Late Latin expānsiōn- (stem of expānsiō) a spreading out. See expanse, -ion

expansional, adjective
antiexpansion, adjective
nonexpansion, noun
overexpansion, noun
preexpansion, noun
reexpansion, noun
self-expansion, noun
superexpansion, noun
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
expansion (ɪkˈspænʃən)
1.  the act of expanding or the state of being expanded
2.  something expanded; an expanded surface or part
3.  the degree, extent, or amount by which something expands
4.  an increase, enlargement, or development, esp in the activities of a company
5.  maths
 a.  the form of an expression or function when it is written as the sum or product of its terms
 b.  the act or process of determining this expanded form
6.  the part of an engine cycle in which the working fluid does useful work by increasing in volume
7.  the increase in the dimensions of a body or substance when subjected to an increase in temperature, internal pressure, etc

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1610s, "anything spread out;" 1640s, "act of expanding," from Fr. expansion, L.L. expansionem (nom. expansio) "a spreading out," from L. expandere (see expand).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

expansion ex·pan·sion (ĭk-spān'shən)

  1. An increase in size.

  2. The spreading out of a structure, such as a tendon.

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
expansion   (ĭk-spān'shən)  Pronunciation Key 
  1. An increase in the volume of a substance while its mass remains the same. Expansion is usually due to heating. When substances are heated, the molecular bonds between their particles are weakened, and the particles move faster, causing the substance to expand.

  2. A number or other mathematical expression written in an extended form. For example, a2 + 2ab + b2 is the expansion of (a + b)2.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
The longer the economy's expansion goes on, the slower productivity growth is
  bound to be.
Growth will be more broad-based, with industrial and agricultural output
  matching services expansion.
In all of those scenarios, there would be less money in the economy that can be
  used for expansion and growth.
More generally, though, the growth of planning is closely tied to the expansion
  of college and university administrations.
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