DDoS stands for Distributed Denial of Service, which refers to the deployment of large numbers of internet bots which can be anywhere from hundreds to hundreds of thousands. These bots are designed to attack a single server, network or application with an overwhelming number of requests, packets or messages, which will deny service to legitimate users such as employees or customers.
According to dictionary.com DDoS pertaining to or being an incident in which a network of computers floods an online resource with high levels of unwanted traffic so that it is inaccessible to legitimate service requests. A method of attacking a computer system by flooding it with so many messages that it is obliged to shut down.
On February 28th 2018 over a thousand anonymous systems (ASNs) across tens of thousands of unique endpoints attacked the GITHUB developer portal with over 1.35 terabits per second of data, making it the largest DDoS attack in history. The site was effectively shut down for about 10 minutes.