Definition
Plain language
A security vulnerability nobody knows about yet and that has no fix available.
As stated in the literature
A previously unknown software vulnerability for which no patch exists at the time of discovery, often valuable to attackers and defenders alike.
Why it matters: Zero-days are uniquely dangerous because defenders have no fix yet, and AI systems that can find or fix them quickly could shift the balance between attackers and defenders.
For example, a zero-day in a web browser might allow a malicious website to silently install spyware before any patch is available.