A security officer (or security guard) is a person who is paid to protect property, assets, or people. They are usually privately and formally employed civilian personnel. Security officers are generally uniformed and act to protect property by maintaining a high-visibility presence to deter illegal and inappropriate actions, observing (either directly, through patrols, or by watching alarm systems or video cameras) for signs of crime, fire or disorder; then taking action and reporting any incidents to their client and emergency services as appropriate.
Until the 1980s, the term watchman was more commonly applied to this function, a usage dating back to at least the Middle Ages in Europe. This term was carried over to North America where it was interchangeable with night-watchman until both terms were replaced with the modern security-based titles. Security guards are sometimes regarded as fulfilling a private policing function.