A Window of Vulnerability in terms of the world of security research exists from the time that the security hole is discovered by someone – be it the software developer, a security researcher, or a malicious player – until the time in which a fix has been released.
During this time the ideal scenario is that the software vendor is made aware of the problem and feverishly works to fix it.
Software developers are typically very quiet about exploits for which there is no fix yet.