1. n. 
One of the layers that constitute the atomic structure of the clay group of layered silicate minerals. The structure of these minerals can consist of two, three or four layers. The silica layer is a plane of silicon dioxide tetrahedra (silicon at the center and oxygen at all four corners of the tetrahedron). Another structural layer is a plane of aluminum hydroxide octahedra (aluminum at the center and hydroxides at all six corners). The tetrahedral and octahedral layers fit one on top of the other, with oxygen atoms being shared as oxide and hydroxide groups.
See: octahedral layer