the operational independent, professionally watching and patrolling of an assigned scope by security forces
A patrol is commonly a group of people like law enforcement officers, military personnel, or private security contractors. These people are assigned to watch over a specific geographic area.
In military, patrol units are sent for gathering information, combat, or both. Outside the military, patrol officers are Police officers are given areas to keep watch over. This is done to know if there are any problems or disputes in the area.
The term need not be so formal, and it applies wider than just humans. Many animals patrol their territory. They may mark the boundary with scent, and challenge any member of the species who enters the territory. This so-called "agonistic behaviour" occurs because resources are limited.