Obligation, n.
Etymology: [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.
2 more senses
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.
Shades of meaning
Fernald has no entry of its own for obligation; he lists it under contract, duty.
Where he writes about it:
Etymologically, duty is that which is owed or due; obligation, that to or by which one is bound; right, that which is correct, straight, or in the direct line of truth and goodness; responsibility, that for which one must answer. — from duty
Duty arises from the nature of things; obligation and responsibility may be created by circumstances, as by one's own promise, or by the acceptance of a trust, etc. — from duty
We speak of a parent's duty, a debtor's obligation; or of a child's duty of obedience, and a parent's responsibility for the child's welfare. — from duty