Why is it called continuous-time frequency?
$\Omega$ is the usual angular frequency in radians per second, and is equal to $2 \pi f$. It is the way to measure frequency for continuous-time signals.

In discrete-time, frequency is measured in radians per sample, and is denoted as $\omega$. Here, a frequency component with $\omega = 2\pi$ is indistinguishable from $\omega = 0$.

$\Omega$ and $\omega$ is the notation in texts such as Oppenheim-Schafer. Other authors use $\theta$ for discrete-time, and the classic $\omega$ for continuous-time signals.