What are Zombie Channels? In the Lightning Network, what is the definition of a zombie channel?

Can they clearly be identified or is it more of a undefined term that is used for channels that are assumed to be obsolete?
I am not aware of a clear definition for the term zombie channels. At least I don't recall the term being used in the lightning rfc. Also I didn't hear lightning devs in conversations to talk about them. Thus I think the semantics are more along your suggestion that they refer to channels which are for some reason obsolete / inactive / unusable. Disclaimet: just because I never saw the term doesn't mean it has no definition