Distributed Denial Of Service (DDOS)

Distributed Denial Of Service, or DDOS, is a type of attack where many computers flood a target, like a website or server, with traffic to overwhelm it and make it unavailable.

Often, the DDOS uses a operation on the target that is long or costly, so that the number of requests generates a large load on the server.

https://en.wikipedia.org/wiki/Denial-of-service_attack

See also What Happens When a PHP Application Is DDoSed: A Guide for Beginners to Advanced Developers and How PHP Vulnerability Leads to DDoS and Malware Attack?.

Related : Security, Attack, Vulnerability, Regexploit