Zero-day

A zero-day is a security weakness that attackers know about and use before the software maker has released a fix. The name means the maker has had zero days to fix it. Zero-days are valuable to criminals and spy agencies, and are often used in targeted attacks before spreading more widely. When a company releases an emergency update for a zero-day, it is often described as "actively exploited", meaning attacks are already happening. You cannot defend against a zero-day yourself before a fix exists, but you can install the fix as soon as it arrives, ideally the same day, and keep automatic updates switched on.