Free US Law Dictionary

Security Officer

A security guard or security officer is usually a privately and formally employed person who is paid to protect property, assets, or people. Often, security officers are 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.

Since at least the Middle Ages in Europe, the term watchman was more commonly applied to this function.