Source : Webster's Revised Unabridged Dictionary (1913)

On \On\, 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:

   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.

            I stood on the bridge at midnight.    --Longfellow.

   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.

            Whosoever shall fall on this stone shall be broken.
                                                  --Matt. xxi.
                                                  44.

   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.

   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.

   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.

   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.

   7. At or in the time of; during; as, on Sunday we abstain
      from labor. See {At} (synonym).

   8. At the time of, conveying some notion of cause or motive;
      as, on public occasions, the officers appear in full dress
      or uniform. Hence, in consequence of, or following; as, on
      the ratification of the treaty, the armies were disbanded.

   9. Toward; for; -- indicating the object of some passion; as,
      have pity or compassion on him.

   10. At the peril of, or for the safety of. ``Hence, on thy
       life.'' --Dryden.

   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.

   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.

             His blood be on us and on our children. --Matt.
                                                  xxvii. 25.

   13. In reference or relation to; as, on our part expect
       punctuality; a satire on society.

   14. Of. [Obs.] ``Be not jealous on me.'' --Shak.

             Or have we eaten on the insane root That takes the
             reason prisoner?                     --Shak.

   Note: Instances of this usage are common in our older
         writers, and are sometimes now heard in illiterate
         speech.

   15. Occupied with; in the performance of; as, only three
       officers are on duty; on a journey.

   16. In the service of; connected with; of the number of; as,
       he is on a newspaper; on a committee.

   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.

   {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.

            They have added the -en plural form on to an elder
            plural.                               --Earle.

            We see the strength of the new movement in the new
            class of ecclesiastics whom it forced on to the
            stage.                                --J. R. Green.

.
      (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}. (Zo["o]l.) 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 sulphureted hydrogen.
      --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}, ardent spirits; -- 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.

Source : WordNet®

on fire
     adj : 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"; "blazing logs in the fireplace";
           "a burning cigarette"; "a flaming crackling fire";
           "houses on fire" [syn: {ablaze(p)}, {afire(p)}, {aflame(p)},
            {aflare(p)}, {alight(p)}, {blazing}, {burning}, {flaming},
            {on fire(p)}]