Zero-day (computing)

Zero-day (computing) is a bit like stumbling upon a secret doorway that somebody forgot to close. It's a security vulnerability, or a weak spot, in computer software that hackers can use to sneak into your device and access your information without you knowing. Nobody has discovered the weak spot yet (it's a "zero-day"), but once somebody knows about it they can use it to get into your stuff. The security companies that make the software will eventually find and fix the weak spot, but until then it's open to anyone who knows about it.