Definitions of obligation:

• noun:   a legal agreement specifying a payment or action and the penalty for failure to comply
• noun:   a personal relation in which one is indebted for a service or favor
• noun:   the state of being obligated to do or pay something

  Example: "He is under an obligation to finish the job"

• noun:   the social force that binds you to your obligations and the courses of action demanded by that force; every opportunity, an obligation