Plain English Meaning
A little sensor sticking out of the aircraft that tells the pilot how cold or warm the air outside is right now.
Full Definition
A small sensor mounted on the exterior of the aircraft that measures the temperature of the ambient air surrounding the airplane and sends that reading to the cockpit display or temperature gauge.
Context Anchor
Seen on aircraft with electronic flight displays, where outside air temperature may be used by the airspeed display and other instrument calculations.