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.
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The Collaborative International Dictionary of English v.0.48:

Road \Road\ (r[=o]), n. [AS. r[=a]d a riding, that on which one
   rides or travels, a road, fr. r[imac]dan to ride. See Ride,
   and cf. Raid.]
   1. A journey, or stage of a journey. [Obs.]
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            With easy roads he came to Leicester. --Shak.
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   2. An inroad; an invasion; a raid. [Obs.] --Spenser.
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   3. A place where one may ride; an open way or public passage
      for vehicles, persons, and animals; a track for travel,
      forming a means of communication between one city, town,
      or place, and another.
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            The most villainous house in all the London road.
                                                  --Shak.
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   Note: The word is generally applied to highways, and as a
         generic term it includes highway, street, and lane.
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   4. [Possibly akin to Icel. rei[eth]i the rigging of a ship,
      E. ready.] A place where ships may ride at anchor at some
      distance from the shore; a roadstead; -- often in the
      plural; as, Hampton Roads. --Shak.
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            Now strike your saile, ye jolly mariners,
            For we be come unto a quiet rode [road]. --Spenser.
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   On the road, or Uponthe road, traveling or passing over a
      road; coming or going; traveling; on the way.
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            My hat and wig will soon be here,
            They are upon the road.               --Cowper.
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   Road agent, a highwayman, especially on the stage routes of
      the unsettled western parts of the United States; -- a
      humorous euphemism. [Western U.S.]
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            The highway robber -- road agent he is quaintly
            called.                               --The century.
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   Road book, a guidebook in respect to roads and distances.


   road kill See roadkill in the vocabulary.

   Road metal, the broken, stone used in macadamizing roads.


   Road roller, a heavy roller, or combinations of rollers,
      for making earth, macadam, or concrete roads smooth and
      compact. -- often driven by steam.

   Road runner (Zool.), the chaparral cock.

   Road steamer, a locomotive engine adapted to running on
      common roads.

   To go on the road, to engage in the business of a
      commercial traveler. [Colloq.]

   To take the road, to begin or engage in traveling.

   To take to the road, to engage in robbery upon the
      highways.
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   Syn: Way; highway; street; lane; pathway; route; passage;
        course. See Way.
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WordNet (r) 3.0 (2006):

on the road
    n 1: travelling about; "they took the show on the road"; "they
         lost all their games on the road" [syn: on the road, on
         tour]