expansion

noun

ex·​pan·​sion ik-ˈspan(t)-shən 

1

: expanse

2

: the act or process of expanding

territorial expansion

economic expansion

expansion of the universe

3

: the quality or state of being expanded

4

: the increase in volume of working fluid (such as steam) in an engine cylinder after cutoff or in an internal combustion engine after explosion

5

a

: an expanded part

b

: something that results from an act of expanding

The book is an expansion of a lecture series.

6

: the result of carrying out an indicated mathematical operation : the expression of a function in the form of a series

expansional

ik-ˈspan(t)-sh(ə-)nəl 

adjective

Synonyms of expansion

• increase
• boost
• addition
• gain

Examples of expansion in a Sentence

The league is undergoing expansion.

the expansion of a lecture series into a book

This book is an expansion of a lecture series.

Recent Examples on the Web

An assembly of planets moves through your ninth house, putting travel, expansion, and higher purpose at the center of your month. —Kirah Tabourn, Condé Nast Traveler, 28 Mar. 2026

Officials and defense analysts have long said that the constraint is not simply funding, but structural limits in a defense industrial base designed for predictable demand rather than rapid wartime expansion. —CBS News, 28 Mar. 2026

In the first teeny fraction of a second after that expansion started, the densities were far higher than in the cores of stars, higher than in neutron stars—actually, very close to what’s needed to create a black hole! —Phil Plait, Scientific American, 27 Mar. 2026

Washington represents the nation’s birth, Jefferson its westward expansion, Roosevelt its economic growth and Lincoln the preservation of the union during the Civil War. —Lauren Schuster, Kansas City Star, 27 Mar. 2026

Word History

First Known Use

1611, in the meaning defined at sense 1

Phrases Containing expansion

• expansion card
• expansion slot

Rhymes for expansion

• mansion
• scansion

Kids Definition

expansion

noun

ex·​pan·​sion ik-ˈspan-chən 

1

: the act of expanding

2

: the state of being expanded

3

: something expanded or a result of expanding

4

: the result of carrying out indicated mathematical operations

the expansion of (a + b)² is a² + 2ab + b²

Medical Definition

expansion

noun

ex·​pan·​sion ik-ˈspan-chən 

1

: the act or process of expanding

2

: the quality or state of being expanded