WAAS is an acronym for Wide Area Augmentation System. It’s an enhancement to GPS in North America that can allow specially equipped GPS receivers to more accurately calculate their position.
WAAS uses a network of ground-based stations that compare their precisely known location with locations calculated from GPS satellite signals. Any differences found can be used to create correction data that's broadcast from WAAS satellites.
WASS can help correct for error caused by distortion of GPS signals as they pass through the ionosphere as well as clock and orbital variations associated with individual GPS satellites.
Originally designed for use by aircraft during flight approaches, it’s now included as a feature in most consumer GPS models.
Similar WADGPS systems that cover other parts of the world include EGNOS (Europe) and MSAS (Japan & Eastern Asia).