Final answer:
An attack that prevents authorized users from accessing a system is known as a denial of service (DoS) attack, which floods the system with excessive traffic.
Step-by-step explanation:
When an attack is designed to prevent authorized users from accessing a system, it is called a denial of service (DoS) attack. This type of cyberattack floods a network or system with traffic to overload resources and bandwidth. As a result, legitimate users are unable to access the system, services, or information they require. An advanced form of this attack is known as a Distributed Denial of Service (DDoS), where the attack comes from a network of compromised computers, making it more difficult to stop.