Law of Closure
The gestalt principle that the mind completes incomplete shapes, perceiving a whole where only parts are shown.
In depth
Closure is why a subject cropped by the frame edge still reads as a complete person, and why a partially hidden shape is understood rather than puzzled over. It gives the photographer licence to show less: a hand and a shadow can imply a whole figure, and the viewer's completion of the picture engages them more than a full description would.
Examples in practice
A silhouette showing only the top half of an arch is read as a complete arch continuing below the frame.