Is Polkadot a ghost chain?

Excuse me, I've been hearing quite a bit about Polkadot in the cryptosphere, but I'm still a bit hazy on its functionality and significance. Could you please clarify for me? Some people seem to refer to it as a 'ghost chain', but I'm not quite sure what that means. Is Polkadot indeed considered a ghost chain? If so, what characteristics or functionalities does it possess that give rise to such a nickname? Or is this just a misnomer, and Polkadot actually serves a much more substantial role in the blockchain ecosystem? I'd really appreciate it if you could break it down for me in a way that's easy to understand. Thank you in advance for your help.