1. duty: something that must be done because of legal or moral duty
2. state of being obligated: the state of being under a moral or legal duty to do something
3. gratitude owed: something that somebody owes in return for something given, e.g. assistance or a favor
4. law binding legal agreement: a legal agreement by which somebody is bound to do something, especially pay a specified amount of money
5. law legal contract: a legal document such as a mortgage or bond that contains the terms of an obligation, usually including a penalty for failing to fulfill it
