The Collaborative International Dictionary of English v.0.48:

On \On\ ([o^]n), prep. [OE. on, an, o, a, AS. on, an; akin to D.
   aan, OS. & G. an, OHG. ana, Icel. [=a], Sw. [*a], Goth. ana,
   Russ. na, L. an-, in anhelare to pant, Gr. 'ana`, Zend ana.
   [root]195. Cf. A-, 1, Ana-, Anon.]
   The general signification of on is situation, motion, or
   condition with respect to contact or support beneath; as:
   [1913 Webster]

   1. At, or in contact with, the surface or upper part of a
      thing, and supported by it; placed or lying in contact
      with the surface; as, the book lies on the table, which
      stands on the floor of a house on an island.
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            I stood on the bridge at midnight.    --Longfellow.
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   2. To or against the surface of; -- used to indicate the
      motion of a thing as coming or falling to the surface of
      another; as, rain falls on the earth.
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            Whosoever shall fall on this stone shall be broken.
                                                  --Matt. xxi.
                                                  44.
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   3. Denoting performance or action by contact with the
      surface, upper part, or outside of anything; hence, by
      means of; with; as, to play on a violin or piano. Hence,
      figuratively, to work on one's feelings; to make an
      impression on the mind.
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   4. At or near; adjacent to; -- indicating situation, place,
      or position; as, on the one hand, on the other hand; the
      fleet is on the American coast.
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   5. In addition to; besides; -- indicating multiplication or
      succession in a series; as, heaps on heaps; mischief on
      mischief; loss on loss; thought on thought. --Shak.
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   6. Indicating dependence or reliance; with confidence in; as,
      to depend on a person for assistance; to rely on; hence,
      indicating the ground or support of anything; as, he will
      promise on certain conditions; to bet on a horse; based on
      certain assumptions.
      [1913 Webster +PJC]

   7. At or in the time of; during; as, on Sunday we abstain
      from labor. See At (synonym).
      [1913 Webster]

   8. At the time of; -- often conveying some notion of cause or
      motive; as, on public occasions, the officers appear in
      full dress or uniform; the shop is closed on Sundays.
      Hence, in consequence of, or following; as, on the
      ratification of the treaty, the armies were disbanded;
      start on the count of three.
      [1913 Webster +PJC]

   9. Toward; for; -- indicating the object of some passion; as,
      have pity or compassion on him.
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   10. At the peril of, or for the safety of. "Hence, on thy
       life." --Dryden.
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   11. By virtue of; with the pledge of; -- denoting a pledge or
       engagement, and put before the thing pledged; as, he
       affirmed or promised on his word, or on his honor.
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   12. To the account of; -- denoting imprecation or invocation,
       or coming to, falling, or resting upon; as, on us be all
       the blame; a curse on him.
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             His blood be on us and on our children. --Matt.
                                                  xxvii. 25.
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   13. In reference or relation to; as, on our part expect
       punctuality; a satire on society.
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   14. Of. [Obs.] "Be not jealous on me." --Shak.
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             Or have we eaten on the insane root
             That takes the reason prisoner?      --Shak.
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   Note: Instances of this usage are common in our older
         writers, and are sometimes now heard in illiterate
         speech.
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   15. Occupied with; in the performance of; as, only three
       officers are on duty; on a journey; on the job; on an
       assignment; on a case; on the alert.
       [1913 Webster +PJC]

   16. In the service of; connected with; a member of; as, he is
       on a newspaper; on a committee.
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   Note: On and upon are in general interchangeable. In some
         applications upon is more euphonious, and is therefore
         to be preferred; but in most cases on is preferable.
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   17. In reference to; about; concerning; as, to think on it;
       to meditate on it.
       [PJC]

   On a bowline. (Naut.) Same as Closehauled.

   On a wind, or On the wind (Naut.), sailing closehauled.


   On a sudden. See under Sudden.

   On board, On draught, On fire, etc. See under Board,
      Draught, Fire, etc.

   On it, On't, of it. [Obs. or Colloq.] --Shak.

   On shore, on land; to the shore.

   On the road, On the way, On the wing, etc. See under
      Road, Way, etc.

   On to, upon; on; to; -- sometimes written as one word,
      onto, and usually called a colloquialism; but it may be
      regarded in analogy with into.
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            They have added the -en plural form on to an elder
            plural.                               --Earle.
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            We see the strength of the new movement in the new
            class of ecclesiastics whom it forced on to the
            stage.                                --J. R. Green.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fire \Fire\ (f[imac]r), n. [OE. fir, fyr, fur AS. f[=y]r; akin
   to D. vuur, OS. & OHG. fiur, G. feuer, Icel. f[=y]ri,
   f[=u]rr, Gr. py^r, and perh. to L. purus pure, E. pure Cf.
   Empyrean, Pyre.]
   1. The evolution of light and heat in the combustion of
      bodies; combustion; state of ignition.
      [1913 Webster]

   Note: The form of fire exhibited in the combustion of gases
         in an ascending stream or current is called flame.
         Anciently, fire, air, earth, and water were regarded as
         the four elements of which all things are composed.
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   2. Fuel in a state of combustion, as on a hearth, or in a
      stove or a furnace.
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   3. The burning of a house or town; a conflagration.
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   4. Anything which destroys or affects like fire.
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   5. Ardor of passion, whether love or hate; excessive warmth;
      consuming violence of temper.
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            he had fire in his temper.            --Atterbury.
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   6. Liveliness of imagination or fancy; intellectual and moral
      enthusiasm; capacity for ardor and zeal.
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            And bless their critic with a poet's fire. --Pope.
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   7. Splendor; brilliancy; luster; hence, a star.
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            Stars, hide your fires.               --Shak.
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            As in a zodiac
            representing the heavenly fires.      --Milton.
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   8. Torture by burning; severe trial or affliction.
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   9. The discharge of firearms; firing; as, the troops were
      exposed to a heavy fire.
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   Blue fire, Red fire, Green fire (Pyrotech.),
      compositions of various combustible substances, as
      sulphur, niter, lampblack, etc., the flames of which are
      colored by various metallic salts, as those of antimony,
      strontium, barium, etc.

   Fire alarm
      (a) A signal given on the breaking out of a fire.
      (b) An apparatus for giving such an alarm.

   Fire annihilator, a machine, device, or preparation to be
      kept at hand for extinguishing fire by smothering it with
      some incombustible vapor or gas, as carbonic acid.

   Fire balloon.
      (a) A balloon raised in the air by the buoyancy of air
          heated by a fire placed in the lower part.
      (b) A balloon sent up at night with fireworks which ignite
          at a regulated height. --Simmonds.

   Fire bar, a grate bar.

   Fire basket, a portable grate; a cresset. --Knight.

   Fire beetle. (Zool.) See in the Vocabulary.

   Fire blast, a disease of plants which causes them to appear
      as if burnt by fire.

   Fire box, the chamber of a furnace, steam boiler, etc., for
      the fire.

   Fire brick, a refractory brick, capable of sustaining
      intense heat without fusion, usually made of fire clay or
      of siliceous material, with some cementing substance, and
      used for lining fire boxes, etc.

   Fire brigade, an organized body of men for extinguished
      fires.

   Fire bucket. See under Bucket.

   Fire bug, an incendiary; one who, from malice or through
      mania, persistently sets fire to property; a pyromaniac.
      [U.S.]

   Fire clay. See under Clay.

   Fire company, a company of men managing an engine in
      extinguishing fires.

   Fire cross. See Fiery cross. [Obs.] --Milton.

   Fire damp. See under Damp.

   Fire dog. See Firedog, in the Vocabulary.

   Fire drill.
      (a) A series of evolutions performed by fireman for
          practice.
      (b) An apparatus for producing fire by friction, by
          rapidly twirling a wooden pin in a wooden socket; --
          used by the Hindoos during all historic time, and by
          many savage peoples.

   Fire eater.
      (a) A juggler who pretends to eat fire.
      (b) A quarrelsome person who seeks affrays; a hotspur.
          [Colloq.]

   Fire engine, a portable forcing pump, usually on wheels,
      for throwing water to extinguish fire.

   Fire escape, a contrivance for facilitating escape from
      burning buildings.

   Fire gilding (Fine Arts), a mode of gilding with an amalgam
      of gold and quicksilver, the latter metal being driven off
      afterward by heat.

   Fire gilt (Fine Arts), gold laid on by the process of fire
      gilding.

   Fire insurance, the act or system of insuring against fire;
      also, a contract by which an insurance company undertakes,
      in consideration of the payment of a premium or small
      percentage -- usually made periodically -- to indemnify an
      owner of property from loss by fire during a specified
      period.

   Fire irons, utensils for a fireplace or grate, as tongs,
      poker, and shovel.

   Fire main, a pipe for water, to be used in putting out
      fire.

   Fire master
      (Mil), an artillery officer who formerly supervised the
            composition of fireworks.

   Fire office, an office at which to effect insurance against
      fire.

   Fire opal, a variety of opal giving firelike reflections.


   Fire ordeal, an ancient mode of trial, in which the test
      was the ability of the accused to handle or tread upon
      red-hot irons. --Abbot.

   Fire pan, a pan for holding or conveying fire, especially
      the receptacle for the priming of a gun.

   Fire plug, a plug or hydrant for drawing water from the
      main pipes in a street, building, etc., for extinguishing
      fires.

   Fire policy, the writing or instrument expressing the
      contract of insurance against loss by fire.

   Fire pot.
      (a) (Mil.) A small earthen pot filled with combustibles,
          formerly used as a missile in war.
      (b) The cast iron vessel which holds the fuel or fire in a
          furnace.
      (c) A crucible.
      (d) A solderer's furnace.

   Fire raft, a raft laden with combustibles, used for setting
      fire to an enemy's ships.

   Fire roll, a peculiar beat of the drum to summon men to
      their quarters in case of fire.

   Fire setting (Mining), the process of softening or cracking
      the working face of a lode, to facilitate excavation, by
      exposing it to the action of fire; -- now generally
      superseded by the use of explosives. --Raymond.

   Fire ship, a vessel filled with combustibles, for setting
      fire to an enemy's ships.

   Fire shovel, a shovel for taking up coals of fire.

   Fire stink, the stench from decomposing iron pyrites,
      caused by the formation of hydrogen sulfide. --Raymond.

   Fire surface, the surfaces of a steam boiler which are
      exposed to the direct heat of the fuel and the products of
      combustion; heating surface.

   Fire swab, a swab saturated with water, for cooling a gun
      in action and clearing away particles of powder, etc.
      --Farrow.

   Fire teaser, in England, the fireman of a steam emgine.

   Fire water, a strong alcoholic beverage; -- so called by
      the American Indians.

   Fire worship, the worship of fire, which prevails chiefly
      in Persia, among the followers of Zoroaster, called
      Chebers, or Guebers, and among the Parsees of India.

   Greek fire. See under Greek.

   On fire, burning; hence, ardent; passionate; eager;
      zealous.

   Running fire, the rapid discharge of firearms in succession
      by a line of troops.

   St. Anthony's fire, erysipelas; -- an eruptive fever which
      St. Anthony was supposed to cure miraculously. --Hoblyn.

   St. Elmo's fire. See under Saint Elmo.

   To set on fire, to inflame; to kindle.

   To take fire, to begin to burn; to fly into a passion.
      [1913 Webster]

WordNet (r) 3.0 (2006):

on fire
    adj 1: lighted up by or as by fire or flame; "forests set ablaze
           (or afire) by lightning"; "even the car's tires were
           aflame"; "a night aflare with fireworks"; "candles alight
           on the tables"; "houses on fire" [syn: ablaze(p),
           afire(p), aflame(p), aflare(p), alight(p), on
           fire(p)]