What does "Zero Day" mean?

I keep seeing this come up

[dvduval]

I found this definition on Google:
Zero Day - A day on which prospective jurors report for jury service but no voir dires actually begin.

What do computer people mean when they say "zero day"?

[Knowles]

end of google dance and they got PR0'ed? Maybe I donno never hear of it.

[jdMorgan]

dvduval,

In the context of timekeeping, zero day is January 1st, 1970.
In Unix systems and others, time is calculated as the number of seconds since the beginning of that day. A simple example related to the web is the time-stamp on cookies.

Jim

[dvduval]

So if I understand correctly, in computer terms, Zero Day might mean the introduction of a new peice of software. Right?

[amznVibe]

software traders use the term Zero-Day to indicate brand new listings/releases which are often highly desired (no comment :) )