Definition
A four-digit transponder code, set on the aircraft's transponder, in which the last two digits are not zero (for example, 1234, 7421, 0537). Discrete codes are assigned individually by ATC to identify a specific aircraft on radar. Codes ending in two zeros (such as 1200, 2000, 7700) are reserved as non-discrete codes for general or emergency use.
Plain English
A unique transponder code ATC gives one specific aircraft so the controller can pick it out from every other target on the radar screen. The last two digits are never both zero, because those codes are reserved for shared or emergency use.
Context Anchor
You encounter this when ATC assigns a squawk code, during radar identification, or when reviewing transponder procedures.
Derivation
Discrete' comes from the Latin discretus, meaning 'separated' or 'distinct.' In this context it means the code is set aside for one individual aircraft — distinct from the shared, general-purpose codes everyone uses by default.
Why Pilots Care
A discrete code lets ATC follow your exact flight path and altitude, which is essential for separation in busy airspace or when flying IFR.
Analogy
It is like being given a numbered badge at an event. The badge does not fly the airplane, but it lets the people watching know exactly which person is you.
Intuition Check
Do not read “discrete” as meaning secret or quiet here. In this context, it means separate and individually identifying.
Example Sentence 1
After contacting departure, the pilot was assigned the discrete code 4263 and squawked it immediately.
Example Sentence 2
Instead of the standard VFR code, we received a discrete code so the controller could keep us separated from other traffic.