A Denial of Service (DoS) describes an effort designed to prohibit or severely limit access to legitimate network resources. The objective is either to restrict access to a network resource or to crash it by overloading the processor. A Distributed Denial of Service is that process synchronized to originate from more than one source.
In 2018, one U.S. target endured a DDoS attack with data requests at around 1.75 terabytes a second in one day. For more information about DDoS and how it has evolved as a cybersecurity threat, see Ars Technica’s write-up of the biggest DDoS attack in US history.