Definition:Element

Definition
An element is a member of a set.

We use the symbol $$x \in S$$ to mean "$$x$$ is an element of the set $$S$$."

Similarly, $$x \notin S$$ means "$$x$$ is not an element of $$S$$."

Some older texts prefer to use $$x \overline \in S$$ for $$x \notin S$$.