Definition:Element/Notation

Definition
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 $x \notin S$.