zero-day
a software flaw the maker does not yet know about
A zero-day is a flaw in software that its maker has not found or fixed. The name means defenders have had zero days to prepare. Because no patch exists, ordinary updates do not protect against it. Attackers who find one can use it until it is discovered and fixed.
Think of it as
A broken latch on a door that nobody, not even the locksmith, knows is broken.