Definition 2.3.1: Ordered and Well-Ordered Set
A set S is called partially ordered if there exists a relation
r (usually denoted by the symbol
)
between S and itself such that the following conditions are satisfied:

- reflexive:
a
a for any element a in S
- transitive: if
a
b and b
c then a
c
- antisymmetric: if
a
b and b
a then a = b
A set S is called well-ordered if it is an ordered set for which every non-empty subset contains a smallest element.