Plesiohedron
In geometry, a plesiohedron is a special kind of space-filling polyhedron, defined as the Voronoi cell of a symmetric Delone set. Three-dimensional Euclidean space can be completely filled by copies of any one of these shapes, with no overlaps. The resulting honeycomb will have symmetries that take any copy of the plesiohedron to any other copy. The plesiohedra include such well-known shapes as the cube, hexagonal prism, rhombic dodecahedron, and truncated octahedron. The largest number of faces that a plesiohedron can have is 38. Definition. A set formula_1 of points in Euclidean space is a Delone  set if there exists a number formula_2 such that every two points of formula_1 are at least at distance formula_4 apart from each other and such that every point of space is within distance formula_5 of at least one point in formula_1. So formula_1 fills space, but its points never come too close to each other. For this to be true, formula_1 must be infinite. Additionally, the set formula_1 is symmetric (in the sense needed to define a plesiohedron) if, for every two points formula_10 and formula_11 of formula_1, there exists a rigid motion of space that takes formula_1 to  formula_1 and formula_10 to formula_11. That is, the symmetries of formula_1 act transitively on formula_1. The Voronoi diagram of any set formula_1 of points partitions space into regions called Voronoi cells that are nearer to one given point of formula_1 than to any other. When formula_1 is a Delone set, the Voronoi cell of each point formula_10 in formula_1 is a convex polyhedron. The faces of this polyhedron lie on the planes that perpendicularly bisect the line segments from formula_10 to other nearby points of formula_1. When formula_1 is symmetric as well as being Delone, the Voronoi cells must all  be congruent to each other, for the symmetries of formula_1 must also be symmetries of the Voronoi diagram. In this case, the Voronoi diagram forms a honeycomb in which there is only a single prototile shape, the shape of these Voronoi cells. This shape is called a plesiohedron. The tiling generated in this way is isohedral, meaning that it not only has a single prototile ("monohedral") but also that any copy of this tile can be taken to any other copy by a symmetry of the tiling. As with any space-filling polyhedron, the Dehn invariant of a plesiohedron is  necessarily zero. Examples. The plesiohedra include the five parallelohedra. These are polyhedra that can tile space in such a way that every tile is symmetric to every other tile by a translational symmetry, without rotation. Equivalently, they are the Voronoi cells of lattices, as these are the translationally-symmetric Delone sets. Plesiohedra are a special case of the stereohedra, the prototiles of isohedral tilings more generally. For this reason (and because Voronoi diagrams are also known as Dirichlet tesselations) they have also been called "Dirichlet stereohedra" There are only finitely many combinatorial types of plesiohedron. Notable individual plesiohedra include: Many  other plesiohedra are known. Two different ones with the largest known number of faces, 38, were discovered by crystallographer Peter Engel. For many years the maximum number of faces of a plesiohedron was an open problem, but analysis of the possible symmetries of three-dimensional space has shown that this number is at most 38. The Voronoi cells of points uniformly spaced on a helix fill space, are all congruent to each other, and can be made to have arbitrarily large numbers of faces. However, the points on a helix are not a Delone set and their Voronoi cells are  not bounded polyhedra. A modern survey is given by Schmitt.