What is Reasonable Suspicion?

Legal Definition
Reasonable suspicion is a legal standard of proof in United States law that is less than probable cause, the legal standard for arrests and warrants, but more than an "inchoate and unparticularized suspicion or 'hunch'"; it must be based on "specific and articulable facts", "taken together with rational inferences from those facts", and the suspicion must be associated with the specific individual. If police additionally have reasonable suspicion that a person so detained is armed and dangerous, they may "frisk" the person for weapons, but not for contraband like drugs. Reasonable suspicion is evaluated using the "reasonable person" or "reasonable officer" standard, in which said person in the same circumstances could reasonably suspect a person has been, is, or is about to be engaged in criminal activity; it depends upon the totality of circumstances, and can result from a combination of particular facts, even if each is individually innocuous.
-- Wikipedia
Legal Definition
Reasonable suspicion is a standard used in criminal procedure. It is looser than probable cause. Reasonable suspicion is sufficient to justify brief stops and detentions, but not enough to justify a full search. When determining reasonable suspicion, courts consider the events leading up to the brief stop and a decide whether these facts, viewed from the standpoint of an objectively reasonable police officer, amount to reasonable suspicion.

Courts look at the totality of the circumstances of each case to see whether the officer has a particularized and objective basis for suspecting legal wrongdoing.