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"
That which one is morally or legally obligated to do.
"As a firefighter, saving lives from burning buildings is his most important duty."
In plain English: A duty is something you are expected to do because it is your responsibility.
"It is my duty to tell the truth, even when it is difficult."
Usage: Use duty to describe a specific responsibility you are expected to fulfill because of your role, job, or moral code. Do not use it to mean a task that is optional or merely convenient.
A surname.
"The judge, whose last name is Duty, reminded everyone that justice must be served without bias."
The word duty comes from the Middle English phrase due plus -ty, which was used to describe something owed or required. It entered the language as a noun meaning an obligation that one is bound to perform.