An Equivalence Relation is a binary relation $\equiv$ on a set A which has the following properties:

For all a, b and c $\epsilon$ A

Once you have an equivalance relation on a set, you can take one element (say, x) and look at all the elements that are equivalent to it. This is called the equivalence class of x.

It's clear that a finite set can be divided up into a finite number of non-empty, disjoint sets, each of which is an equivalence class (of some element). Most people would agree that a countably infinite set can also be divided up into equivalence classes.

It's less clear that dividing up an uncountably infinite set can always be accomplished. This has a connection with the Axiom of Choice.

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