The N-dimensional generalization of region quad/oc-trees.
Tpntrees are quadtrees, octrees and the same thing at any other dimension.
tpn is an acronym for two power n or written as math 2^N which indicates the number of regions a tree has as children.
In 2D its 2^2 = 4, a.k.a. quadtree, in 3D its 2^3 = 8 a.k.a. octree.
For usage information please head over to the docs.