Source : Webster's Revised Unabridged Dictionary (1913)

Obligation \Ob"li*ga"tion\, n. [F. obligation. L. obligatio. See
   {Oblige}.]
   1. The act of obligating.

   2. That which obligates or constrains; the binding power of a
      promise, contract, oath, or vow, or of law; that which
      constitutes legal or moral duty.

            A tender conscience is a stronger obligation than a
            proson.                               --Fuller.

   3. Any act by which a person becomes bound to do something to
      or for anouther, or to forbear something; external duties
      imposed by law, promise, or contract, by the relations of
      society, or by courtesy, kindness, etc.

            Every man has obligations which belong to his
            station. Duties extend beyond obligation, and direct
            the affections, desires, and intentions, as well as
            the actions.                          --Whewell.

   4. The state of being obligated or bound; the state of being
      indebted for an act of favor or kindness; as, to place
      others under obligations to one.

   5. (Law) A bond with a condition annexed, and a penalty for
      nonfulfillment. In a larger sense, it is an acknowledgment
      of a duty to pay a certain sum or do a certain things.

   {Days of obligation}. See under {Day}.

Source : WordNet®

obligation
     n 1: the social force that binds you to your obligations and the
          courses of action demanded by that force; "we must
          instill a sense of duty in our children"; "every right
          implies a responsibility; every opportunity, an
          obligation; every possession, a duty"- John
          D.Rockefeller Jr [syn: {duty}, {responsibility}]
     2: the state of being obligated to do or pay something; "he is
        under an obligation to finish the job"
     3: a personal relation in which one is indebted for a service
        or favor
     4: a legal agreement specifying a payment or action and the
        penalty for failure to comply