noun (n)

• the social force that binds you to the courses of action demanded by that force(noun.act)

  Example:
  Every right implies a responsibility; every opportunity, an obligation; every possession, a duty.
  

  Synonym:
  duty, responsibility
  

  source: wordnet30
• the state of being obligated to do or pay something(noun.state)

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

  source: wordnet30
• a personal relation in which one is indebted for a service or favor(noun.linkdef)

  Synonym:
  indebtedness
  

  source: wordnet30
• a written promise to repay a debt(noun.possession)

  Synonym:
  certificate of indebtedness, debt instrument
  

  source: wordnet30
• a legal agreement specifying a payment or action and the penalty for failure to comply(noun.communication)

  source: wordnet30
• The act of obligating.(noun)

  source: webster1913