A malware attack that takes place immediately after a vulnerability is discovered and before a patch is deployed to the OS, Web browser or application. When a vulnerability is discovered by a user, it often winds up on one or two blogs, and the news travels fast. If only the software vendor becomes aware of it, the tendency is to keep the problem under wraps until a patch has been created to fix it. However, in many cases, vendors announce the flaw so that users can steer clear of the infected website, application or email attachment. See exploit
2021 - A Banner Year
According to MIT Technology Review, there were at least 66 serious zero-day exploits by October 2021, more than double that of 2020. While alarming, researchers also commented that newer safeguards were better at actually detecting more exploits, making it a good sign.