The ∉ (not element of) symbol is used in set theory to denote that a particular element does not belong to a specific set. It could be used in an expression like this:
In plain language, this means that the element does not belong to the set .
The element of symbol indicates that an expression belongs to or is in the set.
The contains as member symbol is used to indicate set membership in set theory.
The does not contain as a member symbol is used in set theory to indicate that an element is not part of a set.