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.
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            Few extend their thoughts toward universal
            knowledge.                            --Locke.
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   2. To enlarge, as a surface or volume; to expand; to spread;
      to amplify; as, to extend metal plates by hammering or
      rolling them.
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   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.
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   4. To hold out or reach forth, as the arm or hand.
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            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.
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   6. To increase in quantity by weakening or adulterating
      additions; as, to extend liquors. --G. P. Burnham.
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   7. (Eng. Law) To value, as lands taken by a writ of extent in
      satisfaction of a debt; to assign by writ of extent.
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   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.
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   Note: This is extended type.

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

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

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)