Planar (two-dimensional) crystallographic group
A planar (or wallpaper) group is a mathematical concept to classify repetitive (periodic) designs on two-dimensional surfaces, such as wallpapers, based on the symmetries in the pattern. Such patterns occur frequently in architecture and decorative art. The mathematical study of the patterns reveals that only 17 different types of them can occur. Wallpaper groups are related to the more complex three-dimensional crystallographic groups (also called space groups).
To read more: List of planar symmetry groups by Wikipedia