Judicial discretion refers to the authority that judges have for making and interpreting certain laws. Within the United States, judicial discretion is one of the fundamental tenants of the system of law, and is guaranteed in the United States Constitution. Both state and federal judges can exercise judicial discretion, although their discretion is not unlimited.
The US Constitution created three branches of government: executive, legislation and judicial. The doctrine of separation of powers vested certain rights in each of these branches. The system of checks and balances ensures that each branch of the government is able to maintain some degree of independence.
These separation of power rules vest some discretion in the judicial branches, which means that judges are guaranteed to be able to exercise discretion by the US Constitution. Judges can use this discretion to decide cases, and to make common law (also called case law) rules where no existing rule applies. Judges can also use their discretion to interpret existing laws, as long as their interpretation does not conflict with the plain language of the existing law.
Judicial discretion is limited by the rights vested in the other branches of the government, and by the doctrine of stare decisis, which means "maintain what has been decided" in the original Latin. This means that a judge does not have the unlimited right to make and interpret laws. Outside of these existing rules and regulations, however, judges can, and do, exercise the power of discretion.
The other branches of the government have the right to make and pass statutes, provided they follow the appropriate guidelines for doing so as set forth in either federal or state constitutions. If a state or the federal government passes a law, it is called statutory law. Judges are bound to follow statutory law, although if a law is not clear, they can interpret it.
Judges therefore cannot use their discretion to overrule statutory law. They can only apply it as they see fit. Although they can't simply change laws, however, the Supreme court does have the discretion to declare that an existing statutory law is unconstitutional.
Stare decisis is the other way in which discretion is limited. Stare decisis means that judges cannot change existing case law interpretations or existing case law rules. This means if another judge has already made a decision on an issue, all future judges must apply it in most cases.
Existing case-made law, or judge-made law can be changed by higher courts, however. This means that even though stare decisis prohibits one judge from coming along and changing what another judge said, a higher court does have the judicial discretion to overturn the existing rule or interpretation. Judges tend to be hesitant to do this because of the strong interest in maintaining precedent, but it does happen on occasion.