<mathematics> A set with a total ordering and no infinite descending chains. A total ordering "<=" satisfies x <= x; x <= y <= z => x <= z; x <= y <= x => x=y; and for all x, y, x <= y or y <= x. In addition, if a set W is well-ordered then all non-empty subsets A of W have a least element, i.e. there exists x in A such that for all y in A, x <= y.
Ordinals are isomorphism classes of well-ordered sets, just as integers are isomorphism classes of finite sets.
(19 Apr 1995)