 We can extend the idea of model-theoretic definition from a single sentence \(S\) to a set \(T\) of sentences; \(\Mod(T)\) is the class of all interpretations that are simultaneously models of all the sentences in \(T\). When a set \(T\) of sentences is used to define a class in this way, mathematicians say that \(T\) is a theory or a set of axioms, and that \(T\) axiomatises the class \(\Mod(T)\).