A rebuttable presumption is a term used in law to reflect a situation where a fact is assumed to be true but may be rebutted with evidence. In criminal law, it is an example of when the onus of proof shifts from the prosecution to the defendant.
The standard of evidence needed to rebut a presumption depends on the substantive law. Usually, a presumption can be rebutted by a preponderance of the evidence. However, the presumption that a patent is valid can be rebutted in court only by clear and convincing evidence.
A classic case of this is the presumptions regarding possession of drugs, and whether "intent to supply" is added to the charge. This usually goes on the circumstances that the drugs were found in and their amount—no evidence of the defendant actually dealing is usually required for "intent to supply" to be added to the charge. For instance, if someone possesses more than half an ounce of marijuana the person is presumed to have the drugs for the purpose of supply, although the threshold amounts might vary from place to place. However, if the accused is able to show (and the onus is on him) that he has that amount for his personal use (i.e., he smokes a lot of weed) then he can escape with just a possession charge.