Plain English Meaning
A number on the GPS that tells you how far you are from a chosen point, measured along the path you are flying. Pilots watch this number to know when to turn in a holding pattern.
Full Definition
The distance value displayed by an Along-Track Distance function on an IFR-certified GPS or FMS, showing how far the aircraft is from a specific waypoint along its programmed flight path. In holding pattern operations, the ATD readout is used to determine when to begin the outbound turn so the inbound leg meets the required timing or distance.
Context Anchor
Seen in instrument procedure and holding-pattern discussions when distance along a or is used to where the aircraft is in relation to a or turn point.