\begin{definition}[Definition:Element/Notation]
The symbol universally used in modern mainstream mathematics to mean '''$x$ is an element of $S$''' is:
:$x \in S$
Similarly, $x \notin S$ means '''$x$ is ''not'' an element of $S$'''.
The symbol can be reversed:
:$S \ni x$ means '''$S$ has $x$ as an element''', that is, $x$ is an element of $S$
but this is rarely seen.
Some texts (usually older ones) use $x \mathop {\overline \in} S$ or $x \mathop {\in'} S$ instead of $x \notin S$.
\end{definition}