Probable cause has no fixed definition. In essence, it means that there is adequate objective information to form a belief that it is more likely than not that a crime has occurred or is occurring and that the suspect is involved. However, there are exceptions, such as when circumstances don't allow adequate time to get a warrant. In the Wardlow case, there was no search in the classic sense. As with probable cause, the definition varies from case to case.
Reasonable suspicion requires less certainty than probable cause but more than a mere hunch.
All we really know for sure is that the police must base their suspicion on facts that can be articulated and not on mere intuition. Suppose Wardlow had stood his ground. Even though it was a high crime area, previous cases have held that the police cannot use that one fact to justify a stop and frisk. It was only because Wardlow took flight that the police succeeded in proving reasonable suspicion. The criminal justice system, translated into plain English.