Longman Dictionary of Contemporary English - obligation

1 a moral or legal duty to do something  (obligation to do sth)  (You can look at the books without any obligation to buy.)  (I have certain obligations to my family.) meet/fulfil an obligation (=do something that is your duty)  (Have the employers met their contractual obligations? | a sense of obligation (=feeling that you ought to do something))  (I helped you because I wanted to, not out of any sense of obligation.) 2 be under an obligation a) to have to do something because it is a legal or moral duty  (be under no obligation to do sth)  (We are invited but we are under no obligation to go. | place sb under an obligation)  (Signing a contract places you under a long-term obligation.) b) to owe someone loyalty, thanks, or money because they have done something for you  (I don't want to be under an obligation to anyone.)