Terry v. Ohio was a case having to do with the 4th Amendment and warrantless searches by police officers. It was decided in 1968. While it is an old case, it is very much in the news today as there is a lawsuit against the New York Police Department for the activities that it conducts under the rule from it conducts under the rule from Terry.
The issue in Terry was whether police could stop and frisk people who seem suspicious even if the police lack probable cause to arrest them. If the police had probable cause, there would be no problem. But in this kind of case, the police see activity that they think is suspicious but which is not clearly illegal enough to give them probable cause to arrest the people they see. In Terry, the Supreme Court gave police the right to stop and frisk people in such circumstances.
Under this ruling, police have the right to stop and frisk people if they have reason to believe that the people might be about to engage in criminal activity or if they believe the people are armed and dangerous. The police cannot, however, stop and frisk people based on hunches. Instead, they need to have a reason that they can state clearly and that is objective and reasonable.
This case, then, says that police can stop and frisk people if the police have identifiable, objective, and reasonable causes to suspect that the people are about to engage in criminal activity.