A set that does not have any members is called an empty set.
⇒ Such sets are represented as {} or using the symbol ∅ (derived from Φ) which has been explicitly developed to designate an empty set.
⇒ Sometimes { ∅ } is also used to designate an empty set.
⇒ An empty set is also called a Void set.
⇒ The cardinality of an empty set is 0.