Plain English Meaning
A small box that reads the airplane's altitude and turns it into a signal the transponder can send out, so controllers on the ground can see how high the aircraft is flying.
Full Definition
A device connected to the aircraft's static pressure system that converts the airplane's pressure altitude into a digital code, which the transponder then transmits to ground radar and other equipment so the aircraft's altitude can be displayed to air traffic control.
Context Anchor
Seen in transponder, traffic display, and Traffic Information System discussions, especially when the aircraft must report altitude to support traffic .