An enhanced mode of SSR that permits selective interrogation and reply capability.
ICAO (2004) doc. 9684 Manual of the Secondary Surveillance Radar (SSR) Systems
Whilst traditional Secondary Surveillance Radar (SSR) stations interrogate all aircraft within their range, Mode S establishes selective and addressed interrogations with aircraft within its coverage. Such selective interrogation improves the quality and integrity of the detection, identification and altitude reporting. These improvements translate into benefits in terms of safety, capacity and efficiency - benefits which are key to supporting the future of the high-traffic density airspace of Europe.
EUROCONTROL, CFMU Glossary, Sept. 2011