A police officer (also known as a policeman or police agent in some forces, particularly in the United Kingdom and other Commonwealth nations) is a warranted law employee of a police force. In the United States, "officer" usually is the formal name of the lowest police rank. In many other countries, "officer" is a generic term not specifying a particular rank, and the lowest rank is often "constable". In some nations the use of the rank "officer" is legally reserved for military personnel. Police officers are generally charged with the apprehension of criminals and the prevention and detection of crime, protection and assistance of the general public, and the maintenance of public order. Police officers may be sworn to an oath, and have the power to arrest people and detain them for a limited time, along with other duties and powers. Some police officers may also be trained in special duties, such as counter-terrorism, surveillance, child protection, VIP protection, civil law enforcement, and investigation techniques into major crime including fraud, rape, murder, and drug trafficking. Although many police officers wear a corresponding uniform, some police officers are plain-clothed in order to dissimulate as ordinary citizens.