A new, unpatched vulnerability which is used to perform an attack. The name "zero-day" comes from the fact that no patch yet exists to mitigate the vulnerability being exploited. Zero-days are sometimes used in trojan horses, rootkits, viruses, worms and other kinds of malware to help them spread to and infect additional computers. Also spelled as "zeroday", "0day" and "0-day."
For more information about zero-days, refer to the blog post Security terms explained: What does Zero Day mean?