Definition
A set of high-intensity flashing or rotating lights required on aircraft to make them visible to other aircraft and to people on the ground. They typically consist of a red rotating beacon and white strobe lights mounted on the fuselage and wingtips. They must be operating any time the aircraft engine is running, and during all flight operations, unless the pilot determines they would be detrimental to safety.
Plain English
Bright flashing lights on the airplane that warn other people the aircraft is active. They get switched on before the engine starts and stay on whenever the engine is running.
Context Anchor
Seen on engine-start and before-start checklists, usually just before the engine is started.
Derivation
"Anti" means against, and "collision" means a crash between two objects. The name says exactly what they are for: lights that help prevent a collision by making the aircraft easy to see.
Why Pilots Care
Required equipment for most flights; they greatly reduce the chance of mid-air or taxiway collisions, especially at night or in busy airspace.
Intuition Check
Anti-collision lights do not physically prevent a collision. They are warning lights that help others notice and avoid the airplane.
Example Sentence 1
Before starting the engine, the pilot turned on the anti-collision lights and called "clear prop."
Example Sentence 2
With the anti-collision beacon flashing, the airplane stood out clearly to other traffic while entering the night traffic pattern.