Source : Webster's Revised Unabridged Dictionary (1913)

   2. That which a person is bound by moral obligation to do, or
      refrain from doing; that which one ought to do; service
      morally obligatory.

            Forgetting his duty toward God, his sovereign lord,
            and his country.                      --Hallam.

   3. Hence, any assigned service or business; as, the duties of
      a policeman, or a soldier; to be on duty.

            With records sweet of duties done.    --Keble.

            To employ him on the hardest and most imperative
            duty.                                 --Hallam.

            Duty is a graver term than obligation. A duty hardly
            exists to do trivial things; but there may be an
            obligation to do them.                --C. J. Smith.

   4. Specifically, obedience or submission due to parents and
      superiors. --Shak.

   5. Respect; reverence; regard; act of respect; homage. ``My
      duty to you.'' --Shak.

   6. (Engin.) The efficiency of an engine, especially a steam
      pumping engine, as measured by work done by a certain
      quantity of fuel; usually, the number of pounds of water
      lifted one foot by one bushel of coal (94 lbs. old
      standard), or by 1 cwt. (112 lbs., England, or 100 lbs.,
      United States).

   7. (Com.) Tax, toll, impost, or customs; excise; any sum of
      money required by government to be paid on the
      importation, exportation, or consumption of goods.

   Note: An impost on land or other real estate, and on the
         stock of farmers, is not called a duty, but a direct
         tax. [U.S.]

   {Ad valorem duty}, a duty which is graded according to the
      cost, or market value, of the article taxed. See {Ad
      valorem}.

   {Specific duty}, a duty of a specific sum assessed on an
      article without reference to its value or market.

   {On duty}, actually engaged in the performance of one's
      assigned task.

Source : WordNet®

on duty
     adj : performing or scheduled for duties; "I'm on from five to
           midnight"; "Naval personnel on duty in Alaska"; "her
           on-duty hours were 11p.m. to 7 a.m." [syn: {on(p)}, {on
           duty(p)}, {on-duty(a)}]