extension

American  

[ik-sten-shuhn] / ɪkˈstɛn ʃən /

noun

• extensions
  plural

1. an act or instance of extending, lengthening, stretching out, or enlarging the scope of something.

   Synonyms:

   dilation, increase, enlargement, expansion

   Antonyms:

   contraction

2. the state of being extended, lengthened, or stretched out.

3. that by which something is extended or enlarged; an addition.

   a four-room extension to a house.

   Synonyms:

   continuation, protraction

4. an additional period of time given one to meet an obligation.

   My term paper wasn't finished so I asked for an extension.

   Synonyms:

   delay

5. something that is expandable or can be extended; an extended object.

   a table with drop-leaf extensions.

6. range or scope of extending; degree of extensiveness; extent.

   the extension of our knowledge.

   Synonyms:

   limit

7. an additional telephone that operates on the principal line.

8. Commerce. a written engagement on the part of a creditor, allowing a debtor further time to pay a debt.

9. Physics. that property of a body by which it occupies space.

10. Anatomy.

    1. the act of straightening a limb.

    2. the position that a limb assumes when it is straightened.

11. Surgery. the act of pulling the broken or dislocated part of a limb in a direction from the trunk, in order to bring the ends of the bone into their natural situation.

12. Also called extent.  Logic. the class of things to which a term is applicable, as “the class of such beings as Plato and Alexander” to which the term “man” is applicable.

13. Mathematics. a function having a domain that includes the domain of a given function and that has the same value as the given function at each point in the domain of the given function.

14. Also called file extension.  Computers. one or more characters at the end of a filename, usually following a period, used to indicate the type of file.

15. Manège. the act of bringing or coming into an extended attitude.

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adjective

1. of or relating to extension courses.

extension British  

/ ɪkˈstɛnʃən /

noun

1. the act of extending or the condition of being extended

2. something that can be extended or that extends another object

3. the length, range, etc, over which something is extended; extent

4. an additional telephone set connected to the same telephone line as another set or other sets

5. a room or rooms added to an existing building

6. a delay, esp one agreed by all parties, in the date originally set for payment of a debt or completion of a contract

7. the property of matter by which it occupies space; size

8. 1. the act of straightening or extending an arm or leg

   2. its position after being straightened or extended

9. med a steady pull applied to a fractured or dislocated arm or leg to restore it to its normal position See also traction

10. 1. a service by which some of the facilities of an educational establishment, library, etc, are offered to outsiders

    2. ( as modifier )

       a university extension course

11. logic

    1. the class of entities to which a given word correctly applies: thus, the extension of satellite of Mars is the set containing only Deimos and Phobos Compare intension

    2. a formal theory that includes among its theorems all the theorems of a given theory

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extension Scientific  

/ ĭk-stĕn′shən /

1. Mathematics A set that includes a given and similar set as a subset.

2. Computer Science A set of characters that follow a filename and are separated from it by a period, used to identify the kind of file.

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Other Word Forms

Derived Forms

• extensionalism
  noun
• extensionality
  noun
• nonextension
  noun
• superextension
  noun
• extensional
  adjective
• extensionless
  adjective
• nonextensional
  adjective
• proextension
  adjective
• extensionally
  adverb

Inflected Forms

Nouns

• extensions
  plural

Etymology

Origin of extension

1350–1400; Middle English < Latin extēnsiōn- (stem of extēnsiō ). See extensive, -ion

Explanation

An extension is the act of getting more time, space, or length, like an extension of a deadline that gives you one more day to turn in a paper, or extensions that make your hair look longer. The noun extension comes from the Latin word extendere, meaning “stretch out.” If you’re a great gymnast, you probably have amazing extension — flexible and long-reaching limbs. If someone meets your sister and assumes you will be exactly like her, you might say, "That's a stretch," meaning this person has, by extension, drawn conclusions about you which are somewhat true but not completely accurate.

Example Sentences

See Examples For:

extension extensions

BMW opened a new plant in Hungary last year, where costs are lower, and Mercedes-Benz unveiled an extension to its Hungarian Kecskemet plant last month, making it the firm's largest in Europe.

From Barron's ● Aug. 25, 2026

BMW opened a new plant in lower-cost Hungary last year and Mercedes-Benz unveiled an extension to its Hungarian Kecskemet plant last month, making it the firm's largest in Europe.

From Barron's ● Aug. 25, 2026

Deere turned down the counteroffer, describing it as costing the company roughly $500 million more than the company’s extension proposal.

From The Wall Street Journal ● Aug. 24, 2026

Despite signing a contract extension in November 2024, the series reveals how close Guardiola came to not renewing his deal.

From BBC ● Aug. 17, 2026

One aspect of the Scientific Revolution consists in the extension of mathematical theories to include sublunary phenomena.

From "The Invention of Science" by David Wootton

Alex Cooper’s Unwell Beverage Co. is shutting down, offering a fresh reminder of the challenges that can face celebrity brand extensions.

From The Wall Street Journal ● Aug. 14, 2026

Axons are extensions of nerve cells that allow neurons to communicate with one another.

From Science Daily ● Aug. 14, 2026

They join safeties Quentin Lake and Kam Curl — both of whom received extensions — to give the Rams one of the strongest units in the league.

From Los Angeles Times ● Jul. 23, 2026

In this scenario, output from the Persian Gulf doesn’t recover until December next year, with the support of oil-pipeline extensions.

From MarketWatch ● Jul. 21, 2026

He applied for extensions on grants, often took months to answer letters, and at one point continued to pay a dead employee’s salary for three months before anyone noticed.

From "The Immortal Life of Henrietta Lacks" by Rebecca Skloot

Related Words

• delay
• development
• expansion
• increase
• postponement