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.
Sometimes lamers will call it Ohday (OH-day)
2. Damn I must've got pwned by some 0day.
<other people> Very interesting, person_1!
... 3 hours later ...
<person_2> OH EM GEE X JUST HAPPENED website
<other people/person_1> That's so not 0day.