(mathematics, geometry, group theory) Any of a class of groups whose finite cases are precisely the finite reflection groups (including the symmetry groups of polytopes), but which are more varied in their infinite cases, and whose range of application encompasses various areas of mathematics.

(geometry, algebra) A Coxeter-Dynkin diagram.

(mathematics) The n×n symmetric matrix with entries m_ij, which can be conveniently encoded by a Coxeter diagram.

Harold Scott MacDonald "Donald" Coxeter (9 February 1907 – 31 March 2003) was a British-Canadian geometer and mathematician.