Definition of Fiction of law
An assumption or supposition of law that something which is or may be false is true, or that a state of facts exists which has never really taken place. An assumption, for purposes of justice, of a fact that does not or may not exist. A rule of law which assumes as true, and will not allow to be disproved, something which is false, but not impossible. Ryan v. Motor Credit Co., 30 N.J.Eq. 531, 23 A.2d 607, 621.
These assumptions are of an innocent or even beneficial character, and are made for the advancement of the ends of justice. They secure this end chiefly by the extension of procedure from cases to which it is applicable to other cases to which it is not strictly applicable, the ground of inapplicability being some difference of an immaterial character.
See also Legal fiction.
Estoppels distinguished. Fictions are to be distinguished from estoppels; an estoppel being the rule by which a person is precluded from asserting a fact by previous conduct inconsistent therewith on his own part or the part of those under whom he claims, or by an adjudication upon his rights which he cannot be allowed to question.
Presumptions distinguished. Fictions are to be distinguished from presumptions of law. By the former, something known to be false or unreal is assumed as true; by the latter, an inference is set up, which may be and probably is true, but which, at any rate, the law will not permit to be controverted. It may also be said that a presumption is a rule of law prescribed for the purpose of getting at a certain conclusion, though arbitrary, where the subject is intrinsically liable to doubt from the remoteness, discrepancy, or actual defect of proofs.
That's the definition of Fiction of law in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.