A software function that monitors a device's physical location and triggers an assigned event when the device enters (or leaves) a designated area (or areas).
The device being geo-fenced must have the ability to determine its location, typically using a technology such as A-GPS.
An example might be a smart-home app that runs in the background on your phone, and automatically turns down your heating/cooling, turns off lights, and locks your doors when you're more than one mile away from home.