The Collaborative International Dictionary of English v.0.48:

Obligation \Ob"li*ga"tion\, n. [F. obligation. L. obligatio. See
   Oblige.]
   1. The act of obligating.
      [1913 Webster]

   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.
      [1913 Webster]

            A tender conscience is a stronger obligation than a
            proson.                               --Fuller.
      [1913 Webster]

   3. Any act by which a person becomes bound to do something to
      or for another, or to forbear something; external duties
      imposed by law, promise, or contract, by the relations of
      society, or by courtesy, kindness, etc.
      [1913 Webster]

            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.
      [1913 Webster]

   4. The state of being obligated or bound; the state of being
      indebted for an act of favor or kindness; -- often used
      with under to indicate being in that state; as, to place
      others under obligations to one.
      [1913 Webster]

   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.
      [1913 Webster]

   Days of obligation. See under Day.

   under obligation, under an obligation. in a state of
      obligation[4].
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

obligation
    n 1: the social force that binds you to 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,
         obligation]
    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 [syn: obligation, indebtedness]
    4: a written promise to repay a debt [syn: debt instrument,
       obligation, certificate of indebtedness]
    5: a legal agreement specifying a payment or action and the
       penalty for failure to comply