I sort of feel like I'm nitpicking here, but actually I'm not. It's the twisting and morphing of long-standing terminology and definitions that I want to help thwart -- and some members of the media along with some businesses are prone to try to twist things to get people's attention.
I think every security administrator will agree that a rootkit is actually a program that grants an unauthorized user access to a system -- typically administrator or root level access.
Hiding a directory or file on a system does not constitute a rootkit. Nevertheless, various media outlets are at it again, using the word 'rootkit' in their headlines when the word 'hidden' would be factually accurate.
The issue at hand again centers around Sony. When software is installed for use with a few of their reportedly discontinued fingerprint scanners a hidden directory is created. Of course anytime you create a hidden directory you can also hide files in it. There's no big revelation there.
So why not just report the facts? -- and when in doubt get a decent IT dictionary, or ask a professional.