A contract can be generally described as an contract that is enforceable at law. Gaius categorized agreements into four groups which are: agreements consensu, spoken agreements, agreements re, and agreements litteris. But this category cannot protect all the agreements, such as pacts and innominate contracts; thus, it is no more used.
Quasi-contract is one of the four groups of responsibility in Justinian's category. The primary situations are negotiorumgestio (conducting of another individual's matters without their authorisation), unfair enrichment, and solutioindebiti.