Plain English Meaning
A helper system that watches GPS for errors and sends corrections to your aircraft, making the position information far more accurate and trustworthy than plain GPS alone.
Full Definition
A system that improves the accuracy, integrity, and availability of GPS signals by using a network of ground reference stations to monitor satellite errors, then broadcasting correction data to aircraft through geostationary satellites. In the United States, the SBAS implementation is the Wide Area Augmentation System (WAAS).