Onus probandi is a Latin phrase meaning "burden of proof." The burden of proof is a legal term that refers to the obligation in a court case for one party to use evidence to convince the person or persons making the decision on the case that their side of the story is true. In most legal systems, onus probandi rests with the plaintiff in civil trials and with the prosecution in criminal trials. Depending on the case being tried, the task of fulfilling the burden of proof can vary from one party simply having more evidence than the opposing party to the accusing party needing to prove their case beyond a reasonable doubt. Different burdens of proof also exist to justify such actions as criminal search and seizure, arrest, or indictment.
In any court trial, there is an implied accepted conclusion before any evidence is presented. Whichever side holds onus probandi must then attempt to shift this accepted conclusion away from the opinion of the opposition to their own side with the use of evidence. The other side holds benefit of assumption, meaning that they don't need to prove their case — they simply have to prove that they are not the guilty party.
The question of which side holds onus probandi is dependent on the type of case. In criminal cases, most legal systems put the burden of proof of the case on the prosecution. Civil cases generally require that the plaintiff, or the party bringing the civil suit to court, proves his or her case.
There are different evidentiary burdens that must be fulfilled by the party holding onus probandi. These burdens also depend on the type of case being tried. The most stringent burden comes in criminal trials, where the prosecution is required to prove that the defendant is guilty beyond a reasonable doubt. On the other hand, civil cases simply require that the plaintiff establish a preponderance of the evidence, which simply means that he or she establishes a better case than the defendant. Clear and convincing evidence is a burden that lies between those two extremes, and often comes into play in the sentencing of a criminal.
Questions of onus probandi also arise in other matters that may lead to court cases. For example, police officers that conduct a search and seizure of an individual must prove that they had a reasonable suspicion that a crime was being committed or was imminent. Grounds to make an arrest or indictment require a more severe burden, as the accusing party must prove that there was probable cause for such action. Probable cause means that there was a fair probability that evidence would eventually be found implicating the accused.