Relating to a security vulnerability or exploit that has not been released or known about by the general public. Often begged for on lists such as "Full-Disclosure" by kiddies.
Sometimes lamers will call it Ohday (OH-day)
1) warez obtained on the day of, or slightly before, its official (legal) release.
2) Information that is not publicly available until it is rapidly disseminated in the online community. Often contains the details of new computer vulnerabilities, allowing exploiters to comprimise computer networks before system administrators have a chance to secure their networks.
Dude, do you have access to any 0-day? I need to get my hands on the newversion of Windows.