a procedural rule under which, once a basic fact is established, the fact-finder also must accept the presumed fact that follows from the basic fact, unless the presumption is rebutted

Presumption that is not conclusive and may be contradicted by evidence.

Usually, every element of a case must be proven to a judge or a jury. The exception is a "presumption", which means that if certain other facts are proven, then another fact can be taken for granted by the judge (or jury). For example, in some states, an adult caught having intercourse with a minor is presumed as having known that the minor was under-age. Most presumptions are "rebuttable", which means that the person against whom the presumption applies may present evidence to the contrary, which then has the effect of nullifying the presumption. This then deprives the person that tried to use the presumption with the advantage of the "free" evidence and makes him present evidence to support the fact which might have been proven by the presumption.

Both in common law and in civil law, a rebuttable presumption (in Latin, praesumptio iuris tantum) is an assumption that is made that is taken to be true unless someone comes forward to contest it and prove otherwise. Rebuttable presumptions in criminal law are somewhat controversial in that they do effectively reverse the presumption of innocence in some cases.