Reasonable doubt, in law terminology, refers to the level of surety that a judge and/or juror must have about whether the defendant committed a crime in order to render a guilty verdict. In the United States and many other jurisdictions, all elements that prove the guilt of a defendant must be proven in court to be true beyond a reasonable doubt. This means that if there is a real question about the truth of a fact that proves guilt, such as a valid question about a defendant's presence at a crime scene, the defendant should be found not guilty.
The concept of reasonable doubt is connected to the idea that a defendant is seen to be innocent until proven guilty, as is the case in the U.S. and many other countries. Under this assumption, proof of guilt must be certain beyond a reasonable doubt in the eyes of the judge and/or jury in order for a guilty verdict to be handed down. Proving facts beyond a reasonable doubt is particularly important in criminal cases because the verdict can lead to serious consequences for the defendant.
In civil cases, by contrast, a preponderance of evidence can be sufficient to settle the case in either party's favor. A preponderance of evidence means that one side of the case has established greater certainty than the other, usually through the presence of a greater quantity of credible evidence or more persuasive evidence. Even a small amount of additional evidence can make the difference in settling a case when only a preponderance of evidence is necessary.
In a criminal case, a jury typically must have an abiding conviction, proved by evidence that can be accepted as true because the facts of the case support it beyond reasonable doubt. This could include eyewitness accounts or other evidence that removes valid questions about guilt. Juries must carefully consider all evidence presented in a trial to ensure that evidence that might lead to a conviction has been proved beyond reasonable doubt.
Another term related to the idea of reasonable doubt is the burden of proof. This refers to which party in a court case carries the responsibility of proving its case. In a criminal trial in the U.S. and many other countries, the burden of proof lies with the prosecution. Instead of the defense needing to prove a defendant's innocence, the prosecutor must prove guilt.
Though which party has the burden of proof appears to be a subtle difference, it makes a major difference in how a trial is carried out. In some countries, a defendant is assumed to be guilty until proven innocent. This places a significant burden of proof on the defendant's legal representation and could result in a conviction if innocence cannot be proved, even if the defendant is, in fact, innocent.