Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

   3. (Logic & Metaph.) Capacity of a concept or general term to
      include a greater or smaller number of objects; --
      correlative of intension.

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

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

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

   5. (Physiol.) The straightening of a limb, in distinction
      from flexion.

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

   {Counter extension}. (Surg.) See under {Counter}.

   {Extension table}, a table so constructed as to be readily
      extended or contracted in length.

Source : WordNet®

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}]
     4: an educational opportunity provided by colleges and
        universities to people who not enrolled as regular
        students [syn: {extension service}, {university extension}]
     5: act of stretching or straightening out a flexed limb [ant: {flexion}]
     6: a string of characters beginning with a period and followed
        by one to three 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: {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}]
     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: {lengthiness}, {prolongation}]
     10: an additional telephone set that is connected to the same
         telephone line [syn: {telephone extension}, {extension
         phone}]
     11: an addition to the length of something [syn: {elongation}]
     12: an addition that extends a main building [syn: {annex}, {annexe},
          {wing}]

Source : Free On-Line Dictionary of Computing

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)