Bright-line rule

In United States constitutional law, a bright-line rule (or bright-line test) is a clearly defined rule or standard, composed of objective factors, which leaves little or no room for varying interpretation. The purpose of a bright-line rule is to produce predictable and consistent results in its application. The term "bright-line" in this sense generally occurs in a legal context.
Bright-line rules are usually standards established by courts in legal precedent or by legislatures in statutory provisions. The US Supreme Court often contrasts bright-line rules with their opposite: balancing tests (or "fine line testing"), where a result depends on weighing several factors—which could lead to inconsistent application of law or reduce objectivity.
Debate in the US.
In the United States, there is much scholarly legal debate between those favoring bright-line rules and those favoring balancing tests. While some legal scholars, such as former Supreme Court Justice Antonin Scalia, have expressed a strong preference for bright-line rules, critics often argue that bright-line rules are overly simplistic and can lead to harsh and unjust results. Supreme Court Justice Stephen Breyer noted that there are circumstances in which the application of bright-line rules would be inappropriate, stating that "no single set of legal rules can ever capture the ever changing complexity of human life."
Examples.
"Miranda v. Arizona" (1966) may be considered establishing a bright-line rule. The majority opinion in that case required law enforcement agents to give a criminal suspect what is now known as a Miranda warning of their “Miranda” rights when the suspect is in custody, and when the suspect is about to be interrogated.