The legal standard under Terry v. Ohio regulating stop and frisk limited searches. This is the actual holding of the Court which should be committed to memory.
"We merely hold today that, where a police officer observes unusual conduct which leads him reasonably to conclude in light of his experience that criminal activity may be afoot and that the persons with whom he is dealing may be armed and presently dangerous, where, in the course of investigating this behavior, he identifies himself as a policeman and makes reasonable inquiries, and where nothing in the initial stages of the encounter serves to dispel his reasonable fear for his own or others' safety, he is entitled for the protection of himself and others in the area to conduct a carefully limited search of the outer clothing of such persons in an attempt to discover weapons which might be used to assault him." [392 U.S. 31]