[Sharlin]

It is a way to divide an area, let's say a square, into three "countries". Each country is connected: it constitutes a contiguous region without enclaves or exclaves. The countries, as usual, are also disjoint: no point is shared between two or more countries (points exactly on the border are not thought to belong to any country). Now, the border between the three countries has a very peculiar property: every point of the border separates all three countries!

That is, a bit more rigorously, no matter what border point you choose, you can always find points belonging to all three countries arbitrarily close to it. In non-pathological real-world borders this can only hold for a finite number of points (say, for instance, the point near Basel where the borders between France, Germany, and Switzerland meet).