1. The act of binding oneself by a social, legal, or moral tie.
2. A social, legal, or moral requirement; such as, a duty, contract, or promise that compels one to follow or avoid a particular course of action.
3. A course of action imposed by society, the law, or one's conscience by which a person is bound or restricted.
4. The constraining power of a promise, contract, law, or sense of duty.
5. In law, a legal agreement stipulating a specified payment or action; especially, if the agreement also specifies a penalty for failure to comply and the document containing the terms of such an agreement.
6. Something owed as payment or in return for a special service or favor for which one is indebted to another.
7. The state, fact, or feeling of being indebted to another for a special service or favor received.
8. Etymology: from Old French obligation (1235), from Laton obligationem, obligatio, "an engaging" or pledging"; literally, "a binding" (but rarely used in this sense), noun of action from obligare.

The meaning is of "binding with promises" or "by law" or "duty". Oblige, with which it is usually confused, means "to do one a favor".