A 0 day means we don't know of it's existence, if it's been previous mentioned and people have failed to patch it then it isn't a 0-day anymore. The term comes

A 0 day means we don't know of it's existence, if it's been previous mentioned and people have failed to patch it then it isn't a 0-day anymore. The term comes from the "zero days to fix". Since we've had time to fix this problem, and failed to, it is no longer a 0-day. Hypothetically, third parties might have been safe from this if they fixed it independently.