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n.
the actions and activities assigned to or required or expected of a person or group; "the function of a teacher"; "the government must do its part"; "play its role"
n.
the social force that binds you to the courses of action demanded by that force; "we must instill a sense of duty in our children"; "every right implies a responsibility; every opportunity, an obligation; every possession, a duty"- John D.Rockefeller Jr
n.
(law) an explanation of the fundamental reasons (especially an explanation of the working of some device in terms of laws of nature); "the rationale for capital punishment"; "the principles of internal-combustion engines"