Zero-day (adjective)

A class of software-security-weakness-issues where independent researchers discover a software flaw before the owners of the code discover it. Zero-day, or 0-day in hacker slang, refers to the moment the race starts, on day zero, between network defenders who are trying to fix the flaw before hackers leverage it to cause damage. It is a race because on day zero, there is no known fix to the issue.