Zero-day

Zero-days is a new, unpatched vulnerability 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 trojans, 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?