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

Thus, as a vision of our transportation system in 2050, this plan is flawed due to placing too much emphasis on freeway expansion and insufficient resources on public transit. —Chris Roberts, San Diego Union-Tribune, 6 Feb. 2026

One possibility is an expansion of the United Nations Truce Supervision Organization, or UNTSO, which maintains a small observer force in Lebanon. —Abby Sewell, Los Angeles Times, 6 Feb. 2026

The Roth Tower expansion is ongoing and is expected to be completed before the start of the 2026 football season. —Matt Murschel, The Orlando Sentinel, 6 Feb. 2026

As a marked return to high heels takes hold, one big push at Sam Edelman is the expansion of the brand’s best-selling Hazel pump, which is getting an update with a new a 105-millimeter heel. —Stephen Garner, Footwear News, 6 Feb. 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