look up any word, like cleveland steamer:
 
2.
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 new version of Windows.
by Evan B March 14, 2005
 
1.
Used by software pirates to describe material (software, music, movies) released the same day it hits stores
"Jesus, my gardener, has all the 0day"
by taktix March 31, 2003
 
3.
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. Full-Disclosure Where teh 0day

2. Damn I must've got pwned by some 0day.
by h4x0r June 22, 2004
 
4.
Generally used when talking about cracked software, cracked and released the same day as the software was legally available.
That copy of DOOM I just got off of Funet is 0day man!
by SomeRandomWhiteGuy May 24, 2003
 
5.
Used in IRC or other chat (in the negative) as a sarcastic comment to something already said or "old news."
<person_1> X just happened, check out website
<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.
by rmb September 03, 2006