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.