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Wiktionary
octree

n. A treelike data structure each of whose nodes has up to eight children, most often used to partition a three-dimensional space by recursively subdivide it.

Wikipedia
Octree

An octree is a tree data structure in which each internal node has exactly eight children. Octrees are most often used to partition a three dimensional space by recursively subdividing it into eight octants. Octrees are the three-dimensional analog of quadtrees. The name is formed from oct + tree, but note that it is normally written "octree" with only one "t". Octrees are often used in 3D graphics and 3D game engines.