Definition of Beyond a Reasonable Doubt
Beyond a reasonable doubt must be proven by the prosecution in a standard criminal case. It requires the jury to be satisfied to a "moral certainty." It does not require that all doubt be eliminated, but rather, the evidence is so conclusive that all reasonable doubts are removed from the mind of the ordinary person.
The state's prosecution is held to this strict standard of proof because they are attempting to take a United States citizen and place them in jail maybe for the rest of their life. For example, in some states second degree murder carries an automatic life sentence. Because of the seriousness of the crime and the subsequent penalties the state is held to a strict standard of proof. If a defendant hires a criminal defense lawyer their goal is to argue the state has not proved the elements of the crime against their client beyond a reasonable doubt. Civil cases have a much lower burden of proof. Many civil cases only require the plaintiff to prove their case through a "preponderance of evidence."