"In its original sense, the term obligation was very technical in nature and applied to the responsibility to pay money owed on certain written documents that were executed under seal. Currently obligation is used in reference to anything that an individual is required to do because of a promise, vow, oath, contract, or law. It refers to a legal or moral duty that an individual can be forced to perform or penalized for neglecting to perform....
In its general and most extensive sense, obligation is synonymous with duty. In a more technical meaning, it is a tie which binds us to pay or to do something agreeably to the laws and customs of the country in which the obligation is made. Just. Inst. 1. 3, t. 14. The term obligation also signifies the instrument or writing by which the contract is witnessed."
https://legal-dictionary.thefreedictionary.com/obligation
