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Cardinality
The cardinality of a set $A$, written $n(A)$, is the number of elements in $A$.
Empty set
Written $\emptyset$, is the set with no elements.
Set
A collection of objects. Each object in the set is called an element (or member) of the set.
Subset
A set $A$ is a subset of $B$ (written $A\subseteq B$) if every element of $A$ is also an element of $B$.