1) Polyhedron 
In geometry, an octahedron (plural: octahedra) is a polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex. A regular octahedron is the dual polyhedron of a cube. It is a rectified tetrahedron. It is a square bipyramid in any of three orthogonal orientations....
• (n.) A solid bounded by eight faces. The regular octahedron is contained by eight equal equilateral triangles.
A polyhedron with 8 faces, each of which is an equilateral triangle. A regular octahedron, whose sides are all equilateral triangles, is one of the Platonic solids; it looks like two square pyramids with their square bases stuck together.
An eight-sided geometric solid and one of the forms inthe Isometric Crystal System. The most common crystal in whichdiamonds occur.
Eight sided polyhedron; all sides are equidimensional and bisect at at the same angle. Minerals shaped as octahedrons belong to the isometric system.
Oc`ta·he'dron noun [ Greek ..., from ... eight-side; ... (for ... eight) + ... seat, base, from ... to sit.] (Geom.) A solid bounded by eight faces. The regular octahedron is contained by eight equal equilateral triangles. 
An octahedron is a regular solid comprised of eight equilateral triangles.
[n] - any polyhedron having eight plane faces
A polyhedron with 8 faces. See also: Polyhedral.
noun any polyhedron having eight plane faces
A polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles.