From the “Law Enforcement Digest” Apr 08,

Defining “Terry Stop”
Police may stop a citizen to investigate with less then probable cause to believe that a crime has been committed. But the stop is permissible only if the officer “has reasonable suspicion, grounded in specific and articulable fact that the person stopped has been or is about to be involved in a crime.”

We (court system) look at the totality of the circumstances known to the officer to decide whether the stop meets these criteria. The level of articulable suspicion necessary to support an investigatory detention is “a substantial possibility that a criminal conduct has occurred or is about to occur.” The reasonableness of a stop is a matter of probability not a matter of certainty. Again, the police may stop a suspect and ask for identification and an explanation of his or her activities if they have a well-founded suspicion of criminal activity.