A binary relation, denoted ≤, among the elements of a set such that a ≤ b and b ≤ c implies a ≤ c, and a ≤ b, b ≤ a implies a =b; it need not be the case that either a ≤ b or b ≤ a. Also known as order relation; partial ordering.

A solid-state transformation in certain solid solutions, in which a random arrangement in the lattice is transformed into a regular ordered arrangement of the atoms with respect to one another; a so-called superlattice is formed.