Definition:Big-Omega Notation/Also known as

$\Omega$ Notation: Also known as
Note that in the Greek alphabet, $\Omega$ is the capital letter counterpart of the minuscule $\omega$.

Hence the former is called big-omega and the latter little-omega.

Some sources, therefore, write $\Omega$ notation as big-$\Omega$ notation, despite the fact that $\Omega$'s "big"-ness is intrinsic.