\begin{definition}[Definition:Relation Isomorphism]
Let $\struct {S_1, \RR_1}$ and $\struct {S_2, \RR_2}$ be relational structures.
Let there exist a bijection $\phi: S_1 \to S_2$ such that:
:$(1): \quad \forall \tuple {s_1, t_1} \in \RR_1: \tuple {\map \phi {s_1}, \map \phi {t_1} } \in \RR_2$
:$(2): \quad \forall \tuple {s_2, t_2} \in \RR_2: \tuple {\map {\phi^{-1} } {s_2}, \map {\phi^{-1} } {t_2} } \in \RR_1$
Then $\struct {S_1, \RR_1}$ and $\struct {S_2, \RR_2}$ are '''isomorphic''', and this is denoted $S_1 \cong S_2$.
The function $\phi$ is called a '''relation isomorphism''', or just an '''isomorphism''', from $\struct {S_1, \RR_1}$ to $\struct {S_2, \RR_2}$.
\end{definition}