When someone uses the internet to cause fear and turmoil by disrupting computer networks, that's cyberterrorism.
Some experts argue that the term cyberterrorism should only be used to describe online actions by known terrorist groups. Most people use it to describe any deliberate large-scale network disruption, especially attacks intended to further ideological or political goals. Malware, computer viruses, and hacking government systems are all examples of cyberterrorism. The prefix cyber- refers generally to computers, from cybernetics, the science of communications and control systems.