The burden of proof (Latin: onus probandi) is a level of proof that a party seeking to prove a fact must reach before it is accepted in a court of law.
In a criminal case the burden of proof is on the prosecution. A defendant is not required to prove his or her innocence. The standard the prosecution must reach is proof of their version of the facts beyond a reasonable doubt. In a civil trial the burden of proof is on the one bringing the case to court, called the plaintiff. In a civil trial the burden of proof is on the plaintiff. The standard that must be met is that the "preponderance of the evidence" (weight of the evidence) is enough to prove their case.