!. 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. 2. The constraining power of a promise, contract, law, or sense of duty: I felt no obligation to offer my advice. 3. Law A document in which a person binds himself or herself to undertake or refrain from doing a particular act. 4. A debt instrument, such as a loan, mortgage, or bond. 5. The state, fact, or feeling of being indebted to another for a special service or favor received.