Glossary
Beyond a reasonable doubt
The standard of proof in criminal cases. The government has to prove the defendant's guilt to a level that leaves no reasonable doubt. The highest standard in U.S. law.
Beyond a reasonable doubt doesn't mean beyond all doubt: that would be impossible. It means the kind of doubt that would cause a reasonable person to hesitate to act in their own important affairs. If a juror has that level of doubt, they have to vote not guilty.
This high standard exists because the consequences of a criminal conviction: loss of liberty, criminal record, sometimes the death penalty: are so severe that the system tolerates letting some guilty defendants go free rather than risk convicting innocent ones.