See more synonyms for extension on Thesaurus.com
  1. an act or instance of extending, lengthening, stretching out, or enlarging the scope of something.
  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.
  4. an additional period of time given one to meet an obligation: My term paper wasn't finished so I asked for an extension.
  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.
  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.Compare intension(def 5).
  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.
  1. of or relating to extension courses.

Origin of extension

1350–1400; Middle English < Latin extēnsiōn- (stem of extēnsiō). See extensive, -ion
Related formsex·ten·sion·al·i·ty, ex·ten·sion·al·ism, nounex·ten·sion·al, adjectiveex·ten·sion·al·ly, adverbex·ten·sion·less, adjectivenon·ex·ten·sion, nounnon·ex·ten·sion·al, adjectivepro·ex·ten·sion, adjectivesu·per·ex·ten·sion, noun

Synonyms for extension

See more synonyms for on Thesaurus.com
1. stretching, expansion, enlargement, increase, dilation. 3. lengthening, protraction, continuation. 4. delay. 6. limit.

Antonyms for extension

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for extension

Contemporary Examples of extension

Historical Examples of extension

British Dictionary definitions for extension


  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
    1. the act of straightening or extending an arm or leg
    2. its position after being straightened or extended
  8. med a steady pull applied to a fractured or dislocated arm or leg to restore it to its normal positionSee also traction (def. 3)
    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
  9. 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 PhobosCompare intension (def. 1a)
    2. conservative extensiona formal theory that includes among its theorems all the theorems of a given theory

Word Origin for extension

C14: from Late Latin extensiō a stretching out; see extend
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for extension

c.1400, from Old French extension (14c.) and directly from Latin extensionem/extentionem (nominative extensio/extentio), noun of action from past participle stem of extendere (see extend). In a concrete sense, "extended portion of something" (a railroad, etc.), from 1852. Telephone sense is from 1906.

Online Etymology Dictionary, © 2010 Douglas Harper

extension in Medicine


  1. The act of straightening or extending a flexed limb.
  2. A pulling or dragging force exerted on a limb in a distal direction.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

extension in Science


  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.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.