A relation R on a set A which is a {partial ordering} i.e. IT is reflexive (xRx), transITive (xRyRz => xRz) and antisymmetric (xRyRx => x=y) and for any two elements x and y in A, eITher x R y or y R x. See also equivalence relation, well-ordered. (1995-02-16)