There are a variety of ways to make this precise.
Let be a set, and let be an equivalence relation on . Then there exists a set , the quotient set of modulo . Given any element of , there is an element of , the equivalence class of modulo . Every element of is of this form. Furthermore, and are equal in if and only if in .
The axiom of quotients is an axiom of set theory which states that the paragraph above is true. It corresponds to the clause in the definition of a pretopos (or in Giraud's axioms for a Grothendieck topos) that every congruence has a coequaliser. In most formulations of set theory, this axiom is not needed; instead, it is a theorem when equivalence classes are defined in one of the ways below.
Again, let be a set, and let be an equivalence relation on . Let be an element of . Then the equivalence class of modulo is the subset of consisting of those elements of that are equivalent to :
Then the quotient set is the collection of these equivalence classes.
We may construct this collection using the power set of ; therefore, this may be done in any elementary topos as well as in such diverse set theories as ZFC, SEAR, and ETCS. This definition of equivalence class is quite natural in material set theory, since it immediately produces a set (assuming that subsets are sets).
In some foundations of mathematics, sets are not fundamental, but are defined as more basic presets (sometimes called types? or, confusingly, sets). By definition, a set (sometimes called a setoid) is a preset equipped with an equivalence (pre)relation.
Once more, let be a set, and let be an equivalence relation on . Then the quotient set is the the underlying preset of equipped with (in place of the original equality on ), and the equivalence class is simply .
So again, let be a set, and let be an equivalence relation on . Then the quotient set is the (higher) groupoid whose objects are the same as those of and with a single morphism from to iff (and none otherwise); is simply again.
Any element of is a representative of its equivalence class . Every equivalence class has at least one representative, and its representatives are all equivalent. The set of representatives is the equivalence class in the material set-theoretic sense.
One usually defines properties of equivalence classes and functions on quotient sets by defining them for an arbitrary representative, then proving that the result is independent of the representative chosen. This does not require the axiom of choice.