Terry v. Ohio, 392 U.S. 1 (1968), was a landmark decision by the United States Supreme Court which held that the Fourth Amendment prohibition on unreasonable searches and seizures is not violated when a police officer stops a suspect on the street and frisks him without probable cause to arrest, if the police officer has a reasonable suspicion that the person has committed, is committing, or is about to commit a crime and has a reasonable belief that the person "may be armed and presently dangerous." (392 U.S. 1, at 30).
For their own protection, police may perform a quick surface search of the person’s outer clothing for weapons if they have reasonable suspicion that the person stopped is armed. This reasonable suspicion must be based on "specific and articulable facts" and not merely upon an officer's hunch. This permitted police action has subsequently been referred to in short as a "stop and frisk," or simply a "Terry frisk". The Terry standard was later extended to temporary detentions of persons in vehicles, known as traffic stops; see Terry stop for a summary of subsequent jurisprudence.
The rationale behind the Supreme Court decision revolves around the understanding that, as the opinion notes, "the exclusionary rule has its limitations." The meaning of the rule is to protect persons from unreasonable searches and seizures aimed at gathering evidence, not searches and seizures for other purposes (like prevention of crime or personal protection of police officers).