A Terry stop, so named from the Supreme Court case Terry v. Ohio, 392 U.S. 1 (1968), is the shorthand description for a temporary investigative detention by police. This is a middle ground between a consensual encounter and a full-blown arrest.
If an officer has reasonable, articulable suspicion of a crime, he may briefly detain and investigate further Although there is no hard and fast rule, this detention may not generally last long.
During this detention, if an officer erasonably believes his safety is in danger, he may perform a “frisk” - a brief pat-down of the suspect’s outer clothing to ensure the suspect is not armed. In balancing the minimal intrusion of such a search (reaching into pockets, for example, is not permitted) against the need for officer safety, the Court concluded that the balance weighed iin favor of permitting the brief search.
Of course, if, during the frisk, the officer discovers probable cause evidence of a crime, he may proceed to further investigate, search, or arrest the suspect.
What is “reasonable, articulable suspicion?” It is a particularized, definable condition, somethign the officer can quantify and explain. It is more than a mere hunch, or an inchoate sense.
For example, in Reid v. Georgia, the Court rejected a DEA agent’s accosting a man who walked through an airport past the baggage claim area, occasionally looking backward in the direction of another man. The agent testified that such tactics were common when smuggling drugs.
In Florida v. Royer, they upheld the detention of a man who purchased airline tickets under a false name (at the time, such an action was legal). The Court said that travelling under an assumed name gave rise to a reasonable, articulable suspicion of wrongdoing.
And in Smith v. Ohio, the Court rejected a detention predicated on a young man carrying a brown paper sack “gingerly”.
Hope that helps.