Rules of evidence determine what types of evidence is admissible, and the trial court judge applies these rules to the case. Generally, to be admissible, the evidence must be relevant) and not outweighed by countervailing considerations (e.g., the evidence is unfairly prejudicial, confusing, a waste of time, privileged, or, among other reasons, based on hearsay).
In federal court, the Federal Rules of Evidence govern whether evidence is admissible. Rule 402 provides that “relevant evidence is admissible” unless the Constitution, statute, or the rules make evidence inadmissible. Common rules of evidence that make relevant evidence inadmissible are: Rule 403, which excludes relevant evidence for prejudice, confusion, or waste of time; Rule 404, which generally excludes character evidence and evidence of other crimes, wrong, or acts; and Rule 802, which excludes hearsay, although there are several exceptions. Each state also has its own rules of evidence for state court proceedings, and many states' rules of evidence follow the Federal Rules of Evidence closely.
[Last updated in November of 2021 by the Wex Definitions Team]