Definitions

Airborne Collision Avoidance Systems (ACAS) equipment provides two types of advisories to pilots: Resolution Advisories (RAs) and Traffic Advisories (TAs). These are defined as follows:

Resolution advisory (RA) An indication given to the flight crew recommending:

a) a manoeuvre intended to provide separation from all threats; or

b) a manoeuvre restriction intended to maintain existing separation.

Corrective RA. A resolution advisory that advises the pilot to deviate from the current flight path.

Preventive RA. A resolution advisory that advises the pilot to avoid certain deviations from the current flight path but does not require any change in the current flight path.

Traffic advisory (TA). An indication given to the flight crew that a certain intruder is a potential threat.

Source: ICAO Annex 10, vol. IV (Aeronautical Telecommunications - Surveillance and Collision Avoidance Systems), Fifth Edition, July 2014 - Definitions

Further Reading

ICAO