Fix a first order language $\mathscr{L}$. Let $t_1,t_2$ denote any two $\mathscr{L}$-terms, then one defines an atomic formula as,
$(1)$ $t_1=t_2$ is an atomic formula.
$(2)$ If $R$ is an $n$-ary relation symbol and $t_1,t_2....t_n$ are terms then $R(t_1,t_2..,t_n)$ is also an atomic formula.
I know one example of this i.e $F(x)=y$ (If we assume that $x,y$ as terms and $F$ be some unary function). I was wondering if there exists some other examples for it.