The rule of law is a legal maxim whereby governmental decisions are made by applying known legal principles. Such a government can be called a nomocracy, from the Greek nomos (law) and kratos (power or rule). The phrase can be traced back to 17th century and was popularized in the 19th century by British jurist A. V. Dicey. The concept was familiar to ancient philosophers such as Aristotle, who wrote "Law should govern". Rule of law implies that every citizen is subject to the law. It stands in contrast to the idea that the ruler is above the law, for example by divine right.

Despite wide use by politicians, judges and academics, the rule of law has been described as "an exceedingly elusive notion" giving rise to a "rampant divergence of understandings ... everyone is for it but have contrasting convictions about what it is."

At least two principal conceptions of the rule of law can be identified: a formalist or "thin" and a substantive or "thick" definition of the rule of law. Formalist definitions of the rule of law do not make a judgment about the "justness" of law itself, but define specific procedural attributes that a legal framework must have in order to be in compliance with the rule of law. Substantive conceptions of the rule of law go beyond this and include certain substantive rights that are said to be based on, or derived from, the rule of law.

A police officer (also known as a policeman or policewoman, and constable in some forces, particularly in the United Kingdom and other Commonwealth nations) is a warranted employee of a police force. In the United States, "officer" 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". Police officers are generally charged with the apprehension of criminals and the prevention and detection of crime, 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, and investigation techniques into major crime, including fraud, rape, murder and drug trafficking.

Responsibilities of a police officer are varied, and may differ greatly from within one political context to another. Typical duties relate to keeping the peace, law enforcement, protection of people and property, and the investigation of crimes. Officers are expected to respond to a variety of situations that may arise while they are on duty. Rules and guidelines dictate how an officer should behave within the community, and in many contexts restrictions are placed on what the uniformed officer wears. In some countries, rules and procedures dictate that a police officer is obliged to intervene in a criminal incident, even if they are off-duty. Police officers in nearly all countries retain their lawful powers, while off duty.