"'Stops' refers to the practice of police officers stopping individuals on the street to question them. In general, police may do this to anyone at any time. But unless and until the police officer tells an individual he or she may not leave, a person stopped is free not to answer questions and to leave. As Supreme Court Justice Harlan said in his opinion in Terry [v. Ohio], ordinarily and unless there are specific facts sufficient to justify the officer’s suspicion that a crime is, has been or may about to be committed, the person stopped has a right to ignore the officer’s questions and walk away. But if the individual may not leave, this is called, as Justice Harlan put it, a 'forcible stop.'
"This is why, when stopped, it is always advisable to ask the police officer politely whether you are free to leave or not. If you are restrained from leaving, you should not resist. But at some point, if the stop is more than brief, you should ask whether you are being arrested. This is because more evidence is required to arrest someone than to stop them for questioning."
Glasser, Ira, "Stop, Question and Frisk: What the Law Says About Your Rights," Drug Policy Alliance (New York, NY: May 2011), p. 4.