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]

Moby Thesaurus II by Grady Ward, 1.0:

333 Moby Thesaurus words for "extension":
   L, access, accession, accessory, accompaniment, accretion, accrual,
   accruement, accumulation, acreage, addenda, addendum, additament,
   addition, additive, additory, additum, adjunct, adjuvant, advance,
   affective meaning, aggrandizement, alternate, alternative school,
   ambit, ampliation, amplification, amplitude, annex, annexation,
   appanage, appendage, appendant, appendix, appreciation,
   appurtenance, appurtenant, area, arm, ascent, attachment, augment,
   augmentation, ballooning, bearing, bigness, blab school, bloating,
   block, boarding school, body, boom, boost, breadth, broadening,
   buildup, bulk, caliber, call box, capacity, central school, coda,
   coin telephone, coloring, coming after, compass, complement,
   comprehensiveness, concomitant, connotation, consecution,
   consecutiveness, consequence, consolidated school, continualness,
   continuance, continuation, continuation school, continuing,
   continuity, continuum, corollary, correspondence school,
   country day school, coverage, crescendo, dame school, day school,
   denotation, deployment, depth, descent, desk telephone,
   development, dial telephone, diameter, dimension, dimensions,
   dispersion, distance, distension, drawing out, drift, edema,
   effect, elevation, ell, elongation, emptiness, empty space,
   endurance, enlargement, essence, evening school, expanse,
   expansion, extensiveness, extent, extrapolation, fanning out,
   field, finishing school, fixture, flare, flood, following, force,
   free school, gain, galactic space, gauge, girth, gist,
   grammatical meaning, greatening, greatness, growth, gush, height,
   hike, hiking, idea, impact, implication, import, increase,
   increment, infinite space, infinity, inflation, informal school,
   intension, interstellar space, jump, largeness, leap, length,
   lengthening, lengthiness, lexical meaning, line, lineage,
   linear measures, lingering, literal meaning, logical sequence,
   long time, longitude, longness, magnification, magnitude,
   maintenance, mass, meaning, measure, measurement, mileage,
   mounting, mouthpiece, multiplication, night school,
   nongraded school, nothingness, offshoot, open classroom school,
   orbit, order, order of succession, outer space, overall length,
   overtone, pay station, pendant, pensionat, perpetuation,
   perpetuity, perseverance, persistence, pertinence, phone, pith,
   platoon school, point, posteriority, postposition,
   practical consequence, private school, procession, procrastination,
   production, productiveness, progress, progression,
   progressive school, proliferation, prolongation, proportion,
   proportions, protraction, public school, public telephone, purport,
   pursuance, purview, push-button telephone, radius, raise, raising,
   range, range of meaning, reach, real meaning, receiver, reference,
   referent, reinforcement, relation, relevance, repetition, rise,
   rotation, run, scale, school without walls, scope,
   semantic cluster, semantic field, sense, sequence, series,
   side effect, side issue, significance, signification, significatum,
   signifie, size, snowballing, space, span, span of meaning,
   spatial extension, special school, sphere, spirit, splay, spread,
   spreading, spreading out, staying power, storefront school,
   straight course, strain, street academy, stretch, stretching,
   stringing out, structural meaning, subjunction, substance,
   succession, successiveness, suffixation, sum, sum and substance,
   summer school, superficial extension, supplement, surface, surge,
   sustained action, sustenance, sweep, swelling, symbolic meaning,
   tailpiece, telephone, telephone booth, telephone engineering,
   telephone extension, telephone mechanics, telephone receiver,
   telephonics, telephony, tenor, tension, totality of associations,
   tract, transferred meaning, transmitter, tumescence,
   unadorned meaning, undergirding, undertone, uninterrupted course,
   union school, unremittingness, up, upping, upsurge, upswing,
   uptrend, upturn, vacation school, value, volume, wall telephone,
   waxing, way, widening, width, wing, yardage



The Free On-line Dictionary of Computing (30 December 2018):

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)


Bouvier's Law Dictionary, Revised 6th Ed (1856):

EXTENSION, comm. law. This term is applied among merchants to signify an
agreement made between a debtor and his creditors, by which the latter, in
order to enable the former, embarrassed in his circumstances, to retrieve
his standing, agree to wait for a definite length of time after their
several claims should become due and payable, before they will demand
payment.
     2. Among the French, a similar agreement is known by the name of
atermoiement. Merl. Rep. mot Atermoiement.