Zero-Day Vulnerability

A “zero-day vulnerability” is an unknown or undiscovered security vulnerability that threat actors/attackers discover before the software provider does and is used to target the software, hardware or firmware with malicious code. The term "Zero-Day" is used when security teams are unaware of their software vulnerability and they’ve had “0” days to work on a security patch or an update to fix the issue. This means vulnerabilities can lay undetected for days, months or years until someone (usually a malicious actor) finds it. A zero-day vulnerability is a potential chink in your armor that exists up until it’s patched or removed, meaning for the entire time it takes to develop, test, and deploy a fix the company and those that use it’s products/services are vulnerable to attack.

A researcher discovers an undocumented memory corruption bug in a popular browser. Since the vendor doesn't know about it yet, it's a zero-day vulnerability.