Glossary
Precedent
A previously decided case that future courts look to for guidance. Some precedent is binding (must be followed), and some is merely persuasive (worth considering but not required).
The common-law system runs on precedent. Lawyers researching a legal issue look for past cases that decided similar questions and use them to predict (or argue) how a court should rule on the current question.
Whether a precedent binds the current court depends on hierarchy and jurisdiction. The U.S. Supreme Court binds every court in the country on federal law. A state supreme court binds every court in that state on state law. A federal Court of Appeals binds the district courts within its Circuit but not those in other Circuits. And no court is bound by its own past decisions in absolute terms: courts can overrule themselves, though they're usually reluctant to.