The Collaborative International Dictionary of English v.0.48:

Extend \Ex*tend"\ ([e^]ks*t[e^]nd"), v. t. [imp. & p. p.
   Extended; p. pr. & vb. n. Extending.] [L. extendere,
   extentum, extensum; ex out + tendere to stretch. See
   Trend.]
   1. To stretch out; to prolong in space; to carry forward or
      continue in length; as, to extend a line in surveying; to
      extend a cord across the street.
      [1913 Webster]

            Few extend their thoughts toward universal
            knowledge.                            --Locke.
      [1913 Webster]

   2. To enlarge, as a surface or volume; to expand; to spread;
      to amplify; as, to extend metal plates by hammering or
      rolling them.
      [1913 Webster]

   3. To enlarge; to widen; to carry out further; as, to extend
      the capacities, the sphere of usefulness, or commerce; to
      extend power or influence; to continue, as time; to
      lengthen; to prolong; as, to extend the time of payment or
      a season of trial.
      [1913 Webster]

   4. To hold out or reach forth, as the arm or hand.
      [1913 Webster]

            His helpless hand extend.             --Dryden.
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   5. To bestow; to offer; to impart; to apply; as, to extend
      sympathy to the suffering.
      [1913 Webster]

   6. To increase in quantity by weakening or adulterating
      additions; as, to extend liquors. --G. P. Burnham.
      [1913 Webster]

   7. (Eng. Law) To value, as lands taken by a writ of extent in
      satisfaction of a debt; to assign by writ of extent.
      [1913 Webster]

   Extended letter (Typog.), a letter, or style of type,
      having a broader face than is usual for a letter or type
      of the same height.
      [1913 Webster]

   Note: This is extended type.

   Syn: To increase; enlarge; expand; widen; diffuse. See
        Increase.
        [1913 Webster]

WordNet (r) 3.0 (2006):

extend
    v 1: extend in scope or range or area; "The law was extended to
         all citizens"; "widen the range of applications"; "broaden
         your horizon"; "Extend your backyard" [syn: widen,
         broaden, extend]
    2: stretch out over a distance, space, time, or scope; run or
       extend between two points or beyond a certain point; "Service
       runs all the way to Cranbury"; "His knowledge doesn't go very
       far"; "My memory extends back to my fourth year of life";
       "The facts extend beyond a consideration of her personal
       assets" [syn: run, go, pass, lead, extend]
    3: span an interval of distance, space or time; "The war
       extended over five years"; "The period covered the turn of
       the century"; "My land extends over the hills on the
       horizon"; "This farm covers some 200 acres"; "The Archipelago
       continues for another 500 miles" [syn: cover, continue,
       extend]
    4: make available; provide; "extend a loan"; "The bank offers a
       good deal on new mortgages" [syn: extend, offer]
    5: thrust or extend out; "He held out his hand"; "point a
       finger"; "extend a hand"; "the bee exserted its sting" [syn:
       exsert, stretch out, put out, extend, hold out,
       stretch forth]
    6: reach outward in space; "The awning extends several feet over
       the sidewalk" [syn: extend, poke out, reach out]
    7: offer verbally; "extend my greetings"; "He offered his
       sympathy" [syn: offer, extend]
    8: extend one's limbs or muscles, or the entire body; "Stretch
       your legs!"; "Extend your right arm above your head" [syn:
       stretch, extend]
    9: expand the influence of; "The King extended his rule to the
       Eastern part of the continent" [syn: extend, expand]
    10: lengthen in time; cause to be or last longer; "We prolonged
        our stay"; "She extended her visit by another day"; "The
        meeting was drawn out until midnight" [syn: prolong,
        protract, extend, draw out]
    11: extend or stretch out to a greater or the full length;
        "Unfold the newspaper"; "stretch out that piece of cloth";
        "extend the TV antenna" [syn: unfold, stretch, stretch
        out, extend]
    12: cause to move at full gallop; "Did you gallop the horse just
        now?" [syn: gallop, extend]
    13: open or straighten out; unbend; "Can we extend the legs of
        this dining table?"
    14: use to the utmost; exert vigorously or to full capacity; "He
        really extended himself when he climbed Kilimanjaro"; "Don't
        strain your mind too much" [syn: strain, extend]
    15: prolong the time allowed for payment of; "extend the loan"
    16: continue or extend; "The civil war carried into the
        neighboring province"; "The disease extended into the remote
        mountain provinces" [syn: carry, extend]
    17: increase in quantity or bulk by adding a cheaper substance;
        "stretch the soup by adding some more cream"; "extend the
        casserole with a little rice" [syn: extend, stretch]

Moby Thesaurus II by Grady Ward, 1.0:

319 Moby Thesaurus words for "extend":
   abide, accord, add to, adjourn, administer, advance, afford,
   aggrandize, allocate, allot, allow, amplify, arch over, attain,
   augment, award, balloon, be lengthy, be prolonged, beef up, belong,
   bestow, bestow on, bestraddle, bestride, bide, bloat, blow up,
   boost, branch, branch out, bridge, broaden, build, build up, bulk,
   bulk out, carry, carry on, carry out, carry to, chatter, come to,
   communicate, confer, continue, continue to be, cover, crane,
   crescendo, dawdle, deal, deal in generalities, deal out, deepen,
   defeat time, defer, defy time, delay, deploy, develop, dilate,
   disentangle, dish out, dispense, disperse, distend, dole, dole out,
   donate, drag on, drag out, draw, draw out, dwell, dwell on,
   ecumenicize, elongate, encompass, endure, enlarge, environ, exalt,
   exist, expand, extend out, extend over, extend to, fan, fan out,
   fatten, fill out, flare, flatten, fork out, generalize, get to,
   gift, gift with, give, give freely, give out, globalize, go, go on,
   go out, go to, grant, greaten, hand out, hang fire, hang over,
   hang up, heap, heighten, help to, hike, hike up, hold, hold forth,
   hold off, hold on, hold out, hold over, hold up, huff, imbricate,
   impart, increase, inflate, issue, jack up, jump up, jut, keep,
   keep alive, keep going, keep on, keep up, lap, lap over, last,
   last long, last out, lavish, lay aside, lay by, lay over, lead to,
   lengthen, lengthen out, let have, let out, lie, lie over,
   linger on, live, live on, live through, magnify, maintain,
   make a generalization, maximize, mete, mete out, multiply,
   never finish, offer, open, open up, outdistance, outlie, outrange,
   outreach, outspread, outstretch, overarch, overgrow, overhang,
   overlap, overlie, override, overrun, pad, parlay, perdure,
   perennate, perorate, perpetuate, persist, pigeonhole, pose,
   postpone, pour, prefer, present, preserve, prevail, proceed,
   procrastinate, produce, proffer, prolong, prolongate, prorogate,
   prorogue, protract, puff, puff up, pull, pump, pump up, push aside,
   put aside, put off, put on ice, put straight, put up, pyramid,
   rain, raise, ramify, range, range out, rarefy, reach, reach out,
   reach to, recess, rectify, remain, render, reserve, retain, run,
   run on, run out, run to, serve, set aside, set by, set straight,
   shell out, shelve, shift off, shingle, shower, sit up, sleep on,
   slip, smooth, snow, snowball, span, speak at length, spin out,
   splay, spraddle, sprangle, sprawl, spread, spread like wildfire,
   spread out, stand, stand on tiptoes, stand over, stand up,
   stave off, stay, stay on, straddle, straggle, straighten,
   straighten out, straighten up, strain, stretch, stretch away to,
   stretch out, stretch to, string out, submit, subsist, sufflate,
   supplement, surround, survive, suspend, sustain, sweep, swell,
   table, take a recess, take in, tarry, tauten, temporize, tender,
   tense, thicken, thrust out, tide over, tighten, tumefy, unbend,
   uncurl, unfold, universalize, unkink, unroll, unsnarl, up, vary,
   vouchsafe, waive, wear, wear well, widen, yield



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

extend

    To add features to a program, especially
   through the use of hooks.

   "Extend" is very often used in the phrase "extend the
   functionality of a program."

   Plug-ins are one form of extension.

   (1997-06-21)