A police officer, also known as an officer, policeman, policewoman, cop, police agent, or a police employee is a warranted law employee of a police force. In most countries, "police officer" is a generic term not specifying a particular rank. In some, the use of the rank "officer" is legally reserved for military personnel.
The word police comes from the Greek politeia meaning government, which came to mean its civil administration. Police officers are those empowered by government to enforce the laws it creates. In The Federalist Papers (#51), James Madison wrote "If men were pure, no government would be necessary."These words apply to those who serve government, including police.
The more general term for the function is law enforcement officer or peace officer. A sheriff is typically the top police officer of a county, with that word coming from the person enforcing law over a shire. A person who has been deputized to serve the function of the sheriff is referred to as the deputy. A common nickname for a police officer is cop. The term copper is originally used in Britain to mean "someone who captures". (In British English the term Cop is recorded (Shorter Oxford Dictionary) in the sense of 'To Capture' from 1704, derived from the Latin 'Capere' via the Old French 'Caper'.) The common myth is that it's a term referring to the police officer's buttons which are made of copper. The term County Mountie is used specifically in reference to county police officers or county sheriff's deputies in the United States. As with Canadian Mounties, the term mountie comes from police who serve while mounted on horseback (see cavalry).