Plain English Meaning
The ground-based equipment that reads the signal coming back from your transponder and turns it into the code and altitude the controller sees on their radar screen.
Full Definition
The portion of the Air Traffic Control Radar Beacon System (ATCRBS) ground equipment that translates the coded reply signals received from an aircraft's transponder into the discrete code and altitude information displayed on the controller's radar scope.
Context Anchor
Seen in radar, transponder, and air traffic control surveillance discussions.
Derivation
From 'de-' (reverse) and 'code.' A decoder reverses the coding process — taking the encoded transponder reply and turning it back into readable information for the controller.