Final answer:
A computer that has been taken over by a hacker is referred to as a botnet. Botnets are used to launch denial of service attacks.
Step-by-step explanation:
A computer that has been taken over by a hacker, typically by using a rootkit, is referred to as a botnet. An army of these are used to launch denial of service (DoS) attacks. A botnet is a network of compromised computers that are under the control of the hacker. The hacker can use the botnet to carry out various malicious activities, such as sending spam emails or conducting large-scale attacks.