When the perspective of a picture appears to draw the viewer's eye towards a central point. An example might be a city street as seen from the center of the road.
Convergence of the background towards a single point is the key. Do not confuse this for any use of perspective, e.g. most checkered floors.
It is possible for an image to contain multiple vanishing points, especially if you're viewing a scene from an angle.
This is an artistic technique, so don't bother with images with a photo background unless it's a major focus of the image. One can assume nature gets it right.
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