Justinian first defines an obligation (obligatio)[1] in his Institutiones, Book 3, section 13 as "a legal bond, with which we are bound by necessity of performing some act according to the laws of our State."[2] He further separates the law of obligations into contracts, delicts, quasi-contracts, and quasi-delicts.
Nowadays, obligation, as applied under civilian law, means a legal tie (vinculum iuris) by which one or more parties (obligants) are bound to perform or refrain from performing specified conduct (prestation).[3] Thus an obligation encompasses both sides of the equation, both the obligor's duty to render prestation and the obligee's right to receive prestation. It differs from the common-law concept of obligation which only encompasses the duty aspect.
Every obligation has four essential requisites otherwise known as the elements of obligation. They are:
the obligor: obligant duty-bound to fulfill the obligation; he who has a duty.
the obligee: obligant entitled to demand the fulfillment of the obligation; he who has a right.
the subject matter, the prestation: the performance to be tendered.
a legal bond, the vinculum juris: the cause that binds or connects the obligants to the prestation.