A police state is a state in which the government maintains strict political or social control over the population without adhering to the rule of law. It is the antithesis of the Rechtsstaat ("legal" or "constitutional" state).
The term usually only refers to a regime which claims that the exercise of political power by the state is not subject to law, although regimes which disregard the law in practice may nevertheless constitute police states as a matter of fact, regardless of whether or not they claim to abide by the rule of law.
In contemporary popular usage, "police state" is often considered to be synonymous with "dictatorship". As it has become a term of opprobrium, the formerly narrowly defined and technical definition of the police state has been expanded in recent decades to sometimes include regimes which do not respect individual rights and freedoms, whether or not these rights and freedoms are enshrined in law. At other times the term may sometimes conveys nothing more than disapproval of the policies of a certain government.
Modern police states usually employ some sort of secret police or similar apparatus, although these elements are not considered essential to the definition. The best-known literary treatment of this sort of police state is George Orwell's novel Nineteen Eighty-Four, which describes a totalitarian régime that uses the threat of constant war as a pretext for subjecting the people to continuous mass surveillance in all aspects of their lives.