What is Duty ?
Duty is (noun) 1. work which you have to do One of his duties is to see that the main doors are locked at night. to be duty bound to do something to be obliged to do something If you have any information relating to this case, you are duty bound to pass it to the police. 2. on duty doing official work which you have to do in a job He’s on duty from 9.00 to 6.00. She’s been on duty all day. 3. a tax which has to be paid
source: Easier English, Student Dictionary Upper Intermediate Level