The Merriam-Webster dictionary defines a police state as one in which a political unit characterized by repressive governmental control of political, economic, and social life usually by an arbitrary exercise of power by police and especially secret police in place of regular operation of administrative and judicial organs of the government according to publicly known legal procedures.
From Wikipedia: The term police state describes a state in which the government exercises rigid and repressive controls over the social, economic and political life of the population. A police state typically exhibits elements of totalitarianism and social control, and there is usually little or no distinction between the law and the exercise of political power by the executive.
Often times, people think of a police state as one in which martial law has been declared, or that a 24-hour lockdown in place. Many others think of Nazi Germany when they hear the term, but think it can’t happen in a modern-day ‘democratic’ country (just for clarification, the Founding Fathers designed this country to be a Constitutional Republic, not a democracy. However, police states do not just happen overnight, a country does not go from being free, or relatively free, to a police state in a day. The noose just gets tighter and tighter slowly enough until one day, you run out of air and realize you are at the mercy of a totalitarian regime.