Geofence

Geofence
A digital representation of space. Often containing either a single point and a radius from that point or a series of points that make up a polygon.
Locative media applications will use geofences as triggers for specific actions. For example, a Twitter search with a geofence will only show Tweets that are inside the radius and point of the search query. Or Foursquare will only show tips which are within a certain radius of a user.