Wikipedia 10K Redux by Reagle from Starling archive. Bugs abound!!!
The standard way of defining a relation R on a set X is as some subset of (X x X). We write aRb if and only if (a,b) is in R. Some particularly important kind of relations are MathematicalFunction''s'', EquivalenceRelation''s'', and partial orders (see PartialOrderedSet).