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

Yet another secondary effect of the Iran war is the expansion of modern drone warfare to the Persian Gulf region. —Sudarsan Raghavan, New Yorker, 28 Apr. 2026

Saudi Arabia’s East-West Petroline pipeline is operating near its 7 million-barrel-per-day capacity, with expansion plans under review. —Emma Bussey, FOXNews.com, 27 Apr. 2026

Its 28-game start matches the expansion 1962 Mets — who lost 120 games — along with 1964 and 1983 for the second-worst in team history behind an 8-20 opening in 1981. —Ronald Blum, Chicago Tribune, 27 Apr. 2026

The last time was in 1962, when the Philadelphia Phillies swept the expansion Mets at the Polo Grounds. —Tim Britton, New York Times, 27 Apr. 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