The sample space is the set all possible outcomes.
A fair coin
When flipping a coin
the sample space is $\left\{\text{Heads, Tails}\right\}$ or $\left\{\text{H, T}\right\}$ abbreviated.
The capital letter
is used for sample space.
$n\left(U\right)$ means the number of elements in $U$.
In this example, $n\left(U\right)=$
.