The Collaborative International Dictionary of English v.0.48:

Extension \Ex*ten"sion\, n. [L. extensio: cf. F. extension. See
   Extend, v. t.]
   1. The act of extending or the state of being extended; a
      stretching out; enlargement in breadth or continuation of
      length; increase; augmentation; expansion.
      [1913 Webster]

   2. (Physics) That property of a body by which it occupies a
      portion of space.
      [1913 Webster]

   3. (Logic & Metaph.)
      (a) Capacity of a concept or general term to include a
          greater or smaller number of objects; -- correlative
          of intension.
      (b) the class or set of objects to which a term refers; --
          contrasted with intension, the logical specification
          which defines members of a class, being the set of
          attributes which are necessary and sufficient to
          recognize an object as a member of the class.
          [1913 Webster]

                The law is that the intension of our knowledge
                is in the inverse ratio of its extension. --Sir
                                                  W. Hamilton.
          [1913 Webster]

                The extension of [the term] plant is greater
                than that of geranium, because it includes more
                objects.                          --Abp.
                                                  Thomson.
          [1913 Webster]

   4. (Surg.) The operation of stretching a broken bone so as to
      bring the fragments into the same straight line.
      [1913 Webster]

   5. (Physiol.) The straightening of a limb, in distinction
      from flexion.
      [1913 Webster]

   6. (Com.) A written engagement on the part of a creditor,
      allowing a debtor further time to pay a debt.
      [1913 Webster]

   Counter extension. (Surg.) See under Counter.

   Extension table, a table so constructed as to be readily
      extended or contracted in length.
      [1913 Webster]

WordNet (r) 3.0 (2006):

extension
    n 1: a mutually agreed delay in the date set for the completion
         of a job or payment of a debt; "they applied for an
         extension of the loan"
    2: act of expanding in scope; making more widely available;
       "extension of the program to all in need"
    3: the spreading of something (a belief or practice) into new
       regions [syn: propagation, extension]
    4: an educational opportunity provided by colleges and
       universities to people who are not enrolled as regular
       students [syn: extension, extension service, university
       extension]
    5: act of stretching or straightening out a flexed limb [ant:
       flexion, flexure]
    6: a string of characters beginning with a period and followed
       by one or more letters; the optional second part of a PC
       computer filename; "most applications provide extensions for
       the files they create"; "most BASIC files use the filename
       extension .BAS" [syn: extension, filename extension,
       file name extension]
    7: the most direct or specific meaning of a word or expression;
       the class of objects that an expression refers to; "the
       extension of `satellite of Mars' is the set containing only
       Demos and Phobos" [syn: reference, denotation,
       extension]
    8: the ability to raise the working leg high in the air; "the
       dancer was praised for her uncanny extension"; "good
       extension comes from a combination of training and native
       ability"
    9: amount or degree or range to which something extends; "the
       wire has an extension of 50 feet" [syn: extension,
       lengthiness, prolongation]
    10: an additional telephone set that is connected to the same
        telephone line [syn: extension, telephone extension,
        extension phone]
    11: an addition to the length of something [syn: elongation,
        extension]
    12: an addition that extends a main building [syn: annex,
        annexe, extension, wing]

The Free On-line Dictionary of Computing (19 January 2023):

extension

   1.  filename extension.

   2.  A feature or piece of code which
   extends a program's functionality, e.g. a plug-in.

   (1997-06-21)