A presumption of fact which can be defeated by persuasive evidence to the contrary.
Usually, every element of a case must be proven to a judge or a jury through some form of evidence, be it a witness or a document.
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) (see the definition of prima facie and res ipsa loquitur).
For example, in some states, an adult caught having sexual 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 otherwise proven by the presumption.