Cardinality

The cardinality of a set is the number of unique/distinct elements. It is denoted by

$|A|=n$ where $n$ is a non-negative integer.

Ex: Given $B=\{1,1,3,5\}$

$|B|=3$

Ex: Given $O=\{\varnothing ,\{\varnothing \},\{1,\varnothing \}\}$

$|O|=3$