a detachment used for security or reconnaissance
"The security team sent out a patrol to check for intruders in the empty warehouse."
the activity of going around or through an area at regular intervals for security purposes
"The police officers began their evening patrol to ensure the safety of the neighborhood."
a group that goes through a region at regular intervals for the purpose of security
"The police patrol walked slowly down the dark street to ensure no criminals were hiding nearby."
A going of the rounds along the chain of sentinels and between the posts, by a guard, usually consisting of three or four men, to insure greater security from attacks on the outposts.
"The guard unit conducted their nightly patrol along the line of sentinels to ensure no enemy forces could breach the outer defenses."
In plain English: A patrol is a group of people who walk around an area to keep it safe and watch for trouble.
"The security patrol walked quietly down the hallway every night to check for intruders."
Usage: As a noun, patrol refers specifically to the group of officers assigned to monitor an area rather than just their individual act of walking it. Use this term when describing the team itself (e.g., "The night patrol arrived") instead of using it as a verb for the action they perform.
To go the rounds along a chain of sentinels; to traverse a police district or beat.
"The security officer walked his daily patrol through the quiet neighborhood, stopping at every street corner to check on the sentinels and ensure no disturbances occurred."
In plain English: To patrol means to move around an area regularly while watching for trouble or danger.
"The security guard will patrol the parking lot at night to keep everyone safe."
The word "patrol" comes from the French patrouille, which originally meant a night-watch involving tramping about. Its roots trace back through Germanic languages to words for walking or treading with one's feet.