Definition:Little-Omega Notation/Also known as

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

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

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