The glossary is a constant work-in-progress. If you have words you’d like to have defined let me know. Or if you have definitions that are messing, let me know that too.
Zero-day -
A zero-day is a hack that has been newly published. The zero refers to the fact that it's published before a fix or patch is ready and available. Once there is a known patch then it just becomes a known vulnerability. Further though, if you don't keep your patches up-to-date then all vulnerabilities are zero-day problems for you.