"Despite developers' goal of delivering a product that works entirely as intended, virtually all software and hardware contain bugs.[7] If a bug creates a security risk, it is called a vulnerability. Vulnerabilities vary in their ability to be exploited by malicious actors. Some are not usable at all, while others can be used to disrupt the device with a denial of service attack. The most valuable allow the attacker to inject and run their own code, without the user being aware of it.[8] Although the term "zero-day" initially referred to the time since the vendor had become aware of the vulnerability, zero-day vulnerabilities can also be defined as the subset of vulnerabilities for which no patch or other fix is available.[9][10][11] A zero-day exploit is any exploit that takes advantage of such a vulnerability.[8]"

That's the definition straight from Wikipedia.