A person is detained when circumstances are such that a reasonable person would believe he is not free to leave.
Police may briefly detain a person if they have reasonable suspicion that the person has committed, is committing, or is about to commit a crime. Many state laws explicitly grant this authority; in Terry v. Ohio, the U.S. Supreme Court established it in all jurisdictions, regardless of explicit mention in state or local laws. Police may conduct a limited search for weapons (known as a “frisk”

if they reasonably suspect that the person to be detained may be armed and dangerous.
Police may question a person detained in a Terry stop, but in general, the detainee is not required to answer.[10] However, many states have “stop and identify” laws that explicitly require a person detained under the conditions of Terry to identify himself to a police officer, and in some cases, provide additional information.
Before Hiibel, it was unresolved whether a detainee could be arrested and prosecuted for refusing to identify himself. Authority on this issue was split among the federal circuit courts of appeal,[11] and the U.S. Supreme Court twice expressly refused to address the question.[12] In Hiibel, the Court held, in a 5-4 decision, that a Nevada “stop and identify” law did not violate the United States Constitution. The Court’s opinion implied that a detainee was not required to produce written identification, but could satisfy the requirement merely by stating his name. Some “stop and identify” laws do not require that a detainee identify himself, but allow refusal to do so to be considered along with other factors in determining whether there is probable cause to arrest.
As of January 2010, the Supreme Court has not addressed the validity of requirements that a detainee provide information other than his name.