Plain English Meaning
A satellite that sits in one spot in the sky from the ground's point of view, because it circles the Earth at exactly the same speed the Earth spins. WAAS uses these satellites to send GPS-correction signals down to aircraft receivers.
Full Definition
A satellite placed in a circular orbit about 22,236 miles above the equator that travels at the same rotational speed as the Earth, so it appears to remain over a fixed point on the ground. In the WAAS network, GEO satellites broadcast correction and integrity messages to GPS receivers, improving position accuracy and providing the integrity assurance required for instrument approaches.
Context Anchor
Seen in WAAS discussions when explaining how improved satellite navigation information gets from the WAAS ground to the aircraft.