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.
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            A tender conscience is a stronger obligation than a
            proson.                               --Fuller.
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   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.
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            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.
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   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.
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   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.
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   Days of obligation. See under Day.

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