Vision cone
An AI agent's field of view modelled as a cone — an angle and a range ahead — within which a target in clear sight counts as detected.
A guard should not see out of the back of its head. The vision cone limits what an agent perceives to a wedge in front of it: a target must fall inside the cone's angle and range, and an unobstructed line of sight must confirm it with a ray before the alarm trips. The concept is the engine of the stealth genre — Metal Gear, Thief, Commandos, Splinter Cell — and is often drawn on the floor so the player can plan a route. It usually feeds hearing and a chain of alert states (suspicious, searching, alarmed) wired into a behaviour tree or state machine.