Final answer:
A Zero-day attack is when a hole is found in software and attackers begin exploiting it immediately. It takes advantage of unknown vulnerabilities and can be highly dangerous.
Step-by-step explanation:
When a hole is found in a web browser or other software, and attackers begin exploiting it the very day it is discovered by the developer, it is known as a Zero-day attack. This type of attack takes advantage of a vulnerability that is unknown to the software developer, giving the attacker a head start in exploiting the system before a fix can be implemented. Zero-day attacks can be highly dangerous because there are no patches or updates available to protect against them.