Plain English Meaning
A way of tracking aircraft by listening to their transponder signals from several ground stations at once and using the slight differences in arrival times to work out exactly where the aircraft is.
Full Definition
A surveillance system that determines an aircraft's position by measuring the tiny differences in time it takes for the aircraft's transponder signal to reach several ground receiver stations spread across a wide area. The system uses these time differences to calculate the aircraft's location, providing radar-like coverage in places where traditional radar is impractical, such as mountainous terrain or remote regions.
Context Anchor
You may encounter WAM in discussions of surveillance, especially in where is by or distance.