Wikipedia 10K Redux by Reagle from Starling archive. Bugs abound!!!

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A partial order <= on a set X is a RelatioN satisfying the following conditions: for every a,b,c in X,

    a <= a                              (reflexive property)
    If a <= b, b <= a, then a = b       (antisymmetric property)
    If a <= b, b <= c, then a <= c      (transitive property)

PartialOrderedSet''''s ("posets") where every two elements have both a GreatestLowerBound and a LeastUpperBound are called LatticEs, and include things like sets, groups, or the like under the partial ordering of inclusion.  Posets which satisfy trichotomy (for all a,b, a<=b or b<=a) are called TotalOrderedSets, and include sets of ordinals.

We write a < b if a <= b and the two are not equal.  Given any two a,b in X, the OpenInterval (a,b) is defined to be the set of all x in X such that a < x < b.  Arbitrary unions of OpenIntervals define the order-TopOlogy of the poset.  Most familiar topologies are built up from objects of this sort, e.g. the RealNumbers.

A map f:X->Y from one poset to another is called an order-HomoMorphism if f(x)<=f(y) when x<=y; if it is bijective, it is called an order-IsoMorphism.  Any order-based property a poset has will also be true in any poset isomorphic to it.  Every poset (X,<=) has a unique dual poset (X,>=).