Equivalence relation/Equivalence class/Definition

Let ${\displaystyle {}R\subseteq M\times M}$ be an equivalence relation, and let ${\displaystyle {}x\in M}$. Then

${\displaystyle {}[x]:={\left\{y\in M\mid (x,y)\in R\right\}}\,}$

is called the equivalence class of ${\displaystyle {}x}$ with respect to ${\displaystyle {}R}$.