an assumption that changes the operation of a law without changing the letter of the law

an assumption that something that is (or may be) false or nonexistent is true or real

In the common law tradition, legal fictions are suppositions of fact taken to be true by the courts of law, but which are not necessarily true. They typically are done to evade archaic rules of procedure or to extend the jurisdiction of the courts in ways that were considered useful, but not strictly authorized by the old rule.