A cyber attack that is done through a vulnerability in a software application that the developer of the software is unaware of and is first discovered by the hacker. The term is used to mean that the software developer had zero days to work on a patch to fix an exploit before the exploit was used. One of the most common applications to have a zero day exploit is a web browser.
John: All the data on my computer was destroyed by a virus.
Bill: Yeah you need to be very careful when you download porn.
John: No, I heard it was done through a zero day exploit on Internet Explorer.
Bill: Oh I see. But why are you using Internet Exploder? Everybody knows that's a piece of shit browser.