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Number of elements
The cardinality of a set is the count of elements it contains. You write $|A|$ to denote the cardinality of set $A$.
If $A = \{1, 2, 3\}$, then $|A| = 3$. If $B = \emptyset$, then $|B| = 0$.
Cardinality tells you the size of a set. In programming, this is like calling $\text{len}()$ or $\text{size}()$ on a collection.
