denial of service (DoS) attack

A Denial of Service or DoS attack is a relatively simple hack designed to prevent authorized users from accessing or using the network or system by flooding it with useless traffic. Similarly, a Distributed Denial of Service (DDoS) attack uses multiple compromised systems to simultaneously flood a network. Because there are multiple sources of the traffic, the DDoS attack can be even more difficult to stop. See also Telephony Denial of Service (TDos).