In geometry, an antiprism is a polyhedron (a 3D shape). It is made of two copies of a regular polygon. These polygons are connected using an alternating band of triangles. To connect a pair of n-sided polygons, 2n triangles are needed. Antiprisms are like prisms, but prisms use parallelograms for the bands at the side.
The regular octahedron is a special kind of antiprism, since all of its faces are equilateral triangles. There are triangles on opposite sides of the octahedron. These triangles are connected by the rest of the triangles, which form an alternating band.