Reasonable suspicion is a legal standard that applies in different criminal-law contexts, most often where searches and seizures are involved. It requires that officers have an objectively reasonable basis for suspecting criminal activity before detaining someone. In addition, before conducting a pat-down, officers must reasonably suspect that a subject is armed and dangerous. Officers can, however, ask people to stop and answer questions without reasonable suspicion. (See What's the difference between an arrest and a detention or "stop and frisk"?)
Reasonable suspicion is a standard lower than probable cause, and it doesn't require anywhere near 50% certainty that the detainee has done something illegal.
REASONABLE SUSPICION IN ACTION
Officer Haulk is downtown, wearing plain clothes at 2:30 in the afternoon. He notices two men, Joe and Calvin, standing at a street corner. Something about them doesn't seem right to him, so he continues to watch them from a distance. He sees Calvin walk down the street, past some stores. Calvin pauses and looks in a store window, then turns around and rejoins Joe. Joe then walks to the same store window, looks in, and returns to Calvin. Joe and Calvin trade off walking to the same window and looking in it until each has done so six times.
Officer Haulk sees a third man walk up to Calvin and Joe and engage them in conversation, then walk away. Calvin and Joe continue their pacing for another ten minutes, then walk in the direction of the third man.
Suspecting that they are "casing a job" before committing a robbery, Officer Haulk approaches the three men and identifies himself as a police officer. He asks for their names and receives a mumbled, unintelligible response. Haulk then grabs Calvin, spins him around, and pats him down; he finds a pistol in Calvin's jacket pocket. He also pats down the other two men, finding another gun on Joe.
Before Calvin's trial for gun possession, his lawyer brings a motion to suppress evidence of the gun in his jacket. The court rules that Officer Haulk was entitled to approach the men and ask them their names. It further finds that, because he limited his search to a pat-down and had a reasonable suspicion that Calvin was armed and dangerous, Haulk was justified in grabbing and searching him. Accordingly, the court denies the motion to suppress. (Terry v. Ohio, 392 U.S. 1 (1968).)