What is Quasi-contract?

Legal Definition
In common law jurisdictions, the law of quasi-contract can be traced to the medieval form of action known as indebitatus assumpsit. In essence, the plaintiff would recover a money sum from the defendant as if the defendant had promised to pay it: that is, as if there were a contract subsisting between the parties. The defendant's promise - her agreement to be bound by the "contract" - was implied by law. The law of quasi-contract was generally used to enforce restitutionary
-- Wikipedia
Legal Definition
An obligation imposed by law to prevent unjust enrichment. Also called a contract implied in law or a constructive contract, a quasi contract may be presumed by a court in the absence of a true contract, but not where a contract—either express or implied in fact—covering the same subject matter already exists.

Because a quasi contract is not a true contract, mutual assent is not necessary, and a court may impose an obligation without regard to the intent of the parties. The remedy is typically restitution or recovery under a theory of quantum meruit. Liability is determined on a case-by-case basis.
Illustrative caselaw
See, e.g. Clay v. Independent School Dist. No. 1 of Tulsa County, 935 P.2d 294 (Okl. 1997).
See also
Legal Definition
A determination by a court about the obligation of one party to another. No actual contract exists but the agreement is similar to a contract. It is based on the conduct of the parties, their relationship and that one party may be come unjustly richer without the courts intervention.
Legal Definition
A term used in the civil law. A quasi-contract is the act of a person, permitted by law, by which he obligates himself towards another, or by which another binds himself to him, without any agreement between them.

2. By article 2272 of the Civil Code of Louisiana, which is translated from article 1371 of the Code Civil, quasi-contracts are defined to be "the lawful and purely voluntary acts of a man, from which there results any obligation whatever to a third person, and sometime a reciprocal obligation between the parties." In contracts, it is the consent of the contracting parties which produces the obligation; in quasi-contracts no consent is required, and the obligation arises from the law or natural equity, on the facts of the case. These acts are called quasi-contracts, because, without being contracts, they bind the parties as contracts do.

3. Quasi-contracts may be multiplied almost to infinity. They are, however, divided into five classes: such "relate to the voluntary and spontaneous management of the affairs of another, without authority; the administration of tutorship; the management of common property; the acquisition of an inheritance; and the payment of a sum of money or other thing by mistake, when nothing was due.

4. - 1. Negotiorum gestio. When a man undertakes of his own accord to manage the affairs of another, the person assuming the agency contracts the tacit engagement to continue it, an& complete it, until the owner shall be in a condition to attend to it himself. The obligation of such a person is, 1st. To act for the benefit of the absentee. 2d. He is commonly answerable for the slightest neglect. 3d. He is bound to render an account of his management. Equity obliges the proprietor, whose business has been well managed, 1st. To comply with the engagements contracted by the manager in his name. 2d. To indemnify the manager in all the engagements he has contracted. 3d. To reimburse him all useful and necessary expenses.

5. - 2. Tutorship or guardianship, is the second kind of quasi-contracts, there being no agreement between the tutor and minor.

6. - 3. When a person has the management of a common property owned by himself and others, not as partners, he is bound to account for the profits, and is entitled to be reimbursed for the expenses which he has sustained by virtue of the quasi-contract which is created by his act, called communio bonorum.

7. - 4. The fourth class is the aditio herreditatis, by which the heir is bound to pay the legatees, who cannot be said to have any contract with him or with the deceased.

8. - 5. Indebiti solutio, or the payment to one of what is not due to him, if made through any mistake in fact, or even in law, entitles him who made the payment to an action against the receiver for repayment, condictio indebiti. This action does not lie, 1. If the sum paid was due ex equitate, or by a natural obligation. 2. If he who made the payment; knew that nothing was due, for qui consulto dat quod non, debebat, proesumitur donare.

9. Each of these quasi-contracts has an affinity with some contract; thus the management of the affairs of another without authority, and tutorship, are compared to a mandate; the community of property, to a partnership; the acquisition of an inheritance, to a stipulation; and the payment of a thing which is not due, to a loan.

10. All persons, even infants and persons destitute of reason, who are consequently incapable of consent may be obliged by the quasi-contract, which results from the act of another, and may also oblige others in their favor; for it is not consent which forms these obligations; they are contracted by the act of another, without any act on our part. The use of reason is indeed required in the person whose act formsthe quasi-contract, but it is not re-quired in the person by whom or in whose favor the obligations which result from it are contracted. For instance, if a person undertakes the business of an infant or a lunatic; this is a quasi-contract, which obliges the infant or the lunatic to the person undertaking his affairs, for what he has beneficially expended, and reciprocally obliges the person to give an account of his administration or management.

11. There is no term in the common law which answers to that of quasi-contract; many quasi-contracts may doubtless be classed among implied contracts; there is, however, a difference between them, which an example will make manifest. In case money should be paid by mistake to a minor, it may be recovered from him by the civil law, because his consent is not necessary to a quasi-contract but by the common law, if it can be recovered, it must be upon an agreement to which the law presumes he has consented, and it is doubtful, upon principle, whether such recovery could be had.

See generally, Just. Inst. b. 3, t. 28 Dig. b. 3, tit. 5; Ayl. Pand. b. 4, tit. 31 1 Bro. Civil Law, 386; Ersk. Pr. Laws of Scotl. b. 3, tit. 3, s. 16; Pardessus, Dr. Com. n. 192, et seq.; Poth. Ob. n. 113, et seq.; Merlin, Rep. Riot Quasi-contract; Menestrier, Lecons Elem. du Droit Civil Romain, liv. 3, tit. 28; Civil Code of Louisiana, b. 3, tit. 5; Code Civil, liv. 3, tit. 4, c. 1.
-- Bouviers Law Dictionary
Legal Definition
In the civil law. A contractual relation arising out of transactions between the parties which give them mutual rights and obligations, but do not involve a specific and express convention or agreement between them. Keener, Quasi Co.ntr. 1; Brackett v. Norton, 4 Conn. 524, 10 Am. Dec. 179; People v. Speir, 77 N. Y. 150; Willard v. Doran. 48 Hun. 402, 1 N. Y. Supp. 5S8; Mc-Sorley v. Faulkner (Com. Pi.) 18 N. Y. Supp. 460; Rnilway Co., v. Gaffney, 65 Ohio St. 104, 6l N. E. 153. Quasi contracts are the lawfui and purely voluntary acts of a man. from which there results any obligation whatever to a third person, and sometimes a reciprocal obligation between the parties. Civ. Code La. art. 2293. Persons who have not contracted with each other are often regarded by the Roman law, under a certain state of facts, as if they had actually concluded a convention between themselves. The legal relation which then takes place between these persons, which has always a similarity to a contract obligation, is therefore termed "obligatio quasi ex contractu." Such a relation arises from the conducting of affairs without authority, (negotiorum gestio,) from the payment of what was not due, (solutio indebiti,) from tutorship and curatorship, and from taking possession of an inheritance. Mackeld. Rom. Law, § 491.
-- Black's Law Dictionary
Legal Definition
An obligation raised by law and wherein there was no meeting of the minds, e. g., the obligation to pay a third party for necessaries furnished a wife or child. See 65 Ohio St. 104, 61 N. E. 152.
-- Ballentine's Law Dictionary