ADS-B stands for Automatic Dependent Surveillance – Broadcast
According to the FAA, Automatic “because it periodically transmits information with no pilot or operator involvement required.”
Dependent “Dependent because the position and velocity vectors are derived from the Global Positioning System (GPS) or other suitable Navigation Systems (i.e., FMS).”
Surveillance because it provides a method of determining 3 dimensional position and identification of aircraft, vehicles, or other assets.
Broadcast because it transmits the information available to anyone with the appropriate receiving equipment.