In mathematics, the notation $~$x \in X$~$ (where $~$\in$~$ is written "\in" in LaTeX) means that $~$X$~$ is a set and $~$x$~$ is an element of that set.
Examples
$~$r \in \mathbb{R}$~$ means that $~$r$~$ is a real number.
https://arbital.com/p/in_notation
by Qiaochu Yuan May 27 2016 updated Aug 27 2016
There's a weird E-looking symbol called \in in LaTeX. What does it mean?
In mathematics, the notation $~$x \in X$~$ (where $~$\in$~$ is written "\in" in LaTeX) means that $~$X$~$ is a set and $~$x$~$ is an element of that set.
$~$r \in \mathbb{R}$~$ means that $~$r$~$ is a real number.