A "Terry Stop" is a stop of a person by law enforcement officers based upon "reasonable suspicion" that a person may have been engaged in criminal activity, whereas an arrest requires "probable cause" that a suspect committed a criminal offense. The name comes from the standards established in a 1968 case, Terry v. Ohio.
The issue in the case was whether police should be able to detain a person and subject him to a limited search for weapons without probable cause for arrest. The Court held that police may conduct a limited search of a person for weapons that could endanger the officer or those nearby, even in the absence of probable cause for arrest and any weapons seized may be introduced in evidence.
When a police officer observes unusual conduct which leads him or her to reasonably suspect criminal activity may be occurring and that the persons with whom he is dealing may be armed and presently dangerous, the officer might approach and briefly detain the subjects for the purpose of conducting a limited investigation. The officer must identify himself or herself as a police officer and may make reasonable inquiries. If after initial investigation the officer still has a reasonable fear for the safety of himself and others, the officer may conduct a carefully limited search of the outer clothing in an attempt to discover weapons that might be used to assault him or her.