Some time ago, I read somewhere how traditional definitions fail in real world since they define precise boundaries depending on whether something follows certain condition or not; however, real world is full of edge cases where an object might seem to satisfy two (near) exclusive definitions.
The solution to this problem was to define things not in terms of boundaries but in terms of central points which will correspond with objects that follow the definitions precisely. The farther you go from the central point, objects will follow the definition less rigidly. Thus, the edge cases will be somewhere midway the two central points.
It's been a few years and now, I can't seem to find what this second type definition is called (neither do I remember where I read this). Any ideas what is this concept called?