Cardinality

We denote the size, or cardinality, of a finite set $S$ by: $\boxed{|S|}$

Note: $|\varnothing |=0$

For example:

• Let $S=\{a,b,c\}$ hence $|S|=3$. It is said that the cardinality of $S$ is 3, or $S$ has 3 elements.
• If $A=\{n\in \mathbb{N}|n\text{ is an odd number between }4\text{ and }24\}$ then $|A|=10$.