A basin of attraction in which every point on the common boundary of that basin and another basin is also a boundary of a third
basin. In other words, no matter how closely a boundary point is zoomed into, all
three basins appear in the picture.

This fractal effect can be seen when four mirrored spheres
touch (Sweet et al. 1999).

Nusse, H. E. and Yorke, J. A. "Basins of Attraction." Science271, 1376-1380, 1996.Poon, L.;
Campos, J.; Ott, E.; and Grebogi, C. "Wada Basin Boundaries in Chaotic Scattering."
Int. J. Bifur. Chaos6, 251-265, 1996.Sweet, D. "Photographs
of Wada Basins." http://www.andamooka.org/~dsweet/Spheres/.Sweet,
D.; Ott, E.; and Yorke, J. A. "Complex Topology in Chaotic Scattering."
Nature399, 315, 1999.