Any experiment with randomness can produce one outcome of the many
possible, denoted by the lowercase Greek letter Omega $\omega$.
The outcome space, denoted by the uppercase Greek letter Omega
$\Omega$, is the set, or collection of all possible outcomes.
A single event is a subset of $\Omega$, commonly denoted by increasing
order of uppercase alphabets, starting with $A$, $B$, etc. The empty set
and the entire set $\Omega$ are both valid subsets.
Note that an outcome $\omega_1$ is different than the event
$A={\omega_1}$, where the former is a single outcome, but the latter is
a set with a single element.
$P(A)$ is the probability that $A$ will occur.
If all $n$ outcomes in $\Omega$ are equally likely, then $P(A)$ is denoted
by $\text{number of outcomes where A occurs}/\text{number of outcomes in
}\Omega$.