Plain English Meaning
A continuous radio message your transponder sends out by itself, telling everyone where you are, how high you are, and how fast you're going. It happens automatically, about once a second, even if no one asks for it.
Full Definition
A longer, automatic broadcast transmitted by a Mode S transponder that includes the aircraft's identity, position, altitude, velocity, and other status data. It is the data format used by ADS-B Out (1090ES) to broadcast aircraft information to ATC ground stations and other aircraft once per second without being interrogated.
Context Anchor
Seen in , transponder equipment descriptions, and ADS-B Out discussions.