Plain English Meaning
A type of radar transponder that lets controllers talk to one specific aircraft at a time using a unique ID, instead of pinging every aircraft in the area together. It also carries extra information like altitude and identity, and supports modern systems such as ADS-B and traffic alerting.
Full Definition
A Secondary Surveillance Radar transponder mode that allows air traffic control to interrogate (query) a specific aircraft individually by its unique 24-bit aircraft address, rather than interrogating all aircraft in range at once. Mode S supports selective addressing, two-way data exchange (datalink), , and unique , and is the foundation for ADS-B and .