Definition
An onboard system that monitors the airspace around an aircraft using transponder signals from nearby traffic, alerts the flight crew to potential mid-air collisions, and in advanced versions issues coordinated instructions to climb or descend to avoid the conflict. ACAS is the international term defined by ICAO; in the United States, the equivalent implementation is known as TCAS (Traffic Alert and Collision Avoidance System).
Plain English
Equipment in the cockpit that watches for other aircraft nearby and warns the crew, or tells them to climb or descend, if a collision looks possible.
Context Anchor
Seen in aircraft equipment descriptions, instrument training, air carrier operations, and discussions of traffic alerts or collision avoidance equipment.
Derivation
Built from plain English words: 'collision avoidance' (preventing two things from hitting each other) applied to aircraft. The term is the international name; the U.S. version is called TCAS. Knowing both labels helps when reading documents from ICAO, the FAA, or foreign authorities.
Why Pilots Care
It adds an automated safety layer that helps prevent collisions when visual scanning is limited or traffic is heavy.
Analogy
It is similar to a collision warning feature in a car: it does not remove the driver’s responsibility, but it gives an urgent warning when another vehicle is getting too close.
Intuition Check
Do not assume this system means the airplane will automatically avoid traffic. It alerts the pilot, and in some systems gives climb or descend guidance, but the pilot is still responsible for seeing, avoiding, and flying the aircraft safely.
Example Sentence 1
The crew received an ACAS resolution advisory to climb and immediately began the climb while notifying ATC.
Example Sentence 2
During the approach the pilot followed the aircraft collision avoidance system guidance to climb and avoid the conflicting traffic.