element

An element or member of a set is any one of the distinct objects that belong to that set.

The binary relation "is an element of", also called set membership, is written "$\in$".
$x\in A$ means that "$x$ is an element of $A$." Equivalently,

• "$x$ is a member of $A$"
• "$x$ belongs to $A$"
• "$x$ is in $A$"
• "$x$ lies in $A$"

The negation of set membership is denoted "$\notin$".
$x\notin A$ means that "$x$ is not an element of $A$".

The function which determines the set membership of an element $x$ in $A$ is the indicator function ${\mathrm{\Phi }}_A(x)$