An algebra in which the entities that make up the algebra can be added, subtracted, and multiplied, with division also being possible except by zero. There are four division algebras: real numbers, complex numbers, quaternions, and octonions. The dimensions of the entities that make up the algebras are 1, 2, 4, and 8, respectively. There is a close relation between division algebras and supersymmetry, with a division algebra in d dimensions being associated with supersymmetry in (d+2) dimensions. It has been suggested that division algebras may also have other important roles in fundamental physics.