noun (n)

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

  Example:
  We must instill a sense of duty in our children.
  Every right implies a responsibility; every opportunity, an obligation; every possession, a duty.
  

  Synonym:
  obligation, responsibility
  

  source: wordnet30
• work that you are obliged to perform for moral or legal reasons(noun.act)

  source: wordnet30
• a government tax on imports or exports(noun.possession)

  Synonym:
  tariff
  

  source: wordnet30
• That which is due; payment.(noun)

  source: webster1913