Definition
The aircraft's pressure altitude — referenced to the standard sea-level setting of 29.92 inches of mercury — automatically transmitted by a Mode C or Mode S transponder to ATC radar in the form of a coded signal.
Plain English
It's the altitude your transponder sends down to ATC. The transponder always reports altitude based on a fixed pressure setting (29.92), not the local altimeter setting you're flying with. ATC's computer corrects for that on their end so they see your real altitude.
Context Anchor
Seen during en route instrument flying when air traffic control uses your transponder altitude readout to help keep aircraft safely apart.
Derivation
Encoded' comes from 'code' — the altitude is converted into a digital signal pattern the transponder can broadcast. The word signals that this isn't a spoken or displayed altitude; it's altitude data packaged for a machine to read.
Why Pilots Care
Ensures controllers receive accurate altitude data for traffic separation and safety.
Intuition Check
Encoded does not mean secret or encrypted here. It means the altitude has been converted into an electronic form that the aircraft can transmit and air traffic control can read.
Example Sentence 1
Center advised the pilot that her encoded altitude was reading 300 feet low, so she was asked to stop altitude squawk until the encoder could be checked.
Example Sentence 2
A discrepancy in encoded altitude prompted the pilot to check the transponder and altimeter settings.