A "zero-day" (or "0Day" in the cybersecurity profession) is a vulnerability in an internet-connected device, network component or piece of software that was just discovered or exposed. Because the developer has just learned of the flaw, it also means an official patch or update to fix the issue hasn't been released, meaning a potential attacker has a headstart on the developer to exploit the vulnerability before the developer can issue a patch to fix the issue.
Learn more about zero-day flaws and exploits here.
Source: cybersecurity.osu.edu and cybersophia.net