Distributed denial-of-service attack DDOS. Happens when a hacker or a number of hackers uses a multitude of compromised systems to attack a single target, causing a denial of service for users of the targeted system.
Think of it as a flood going to a single funnel. The flood of incoming messages to the targeted system shuts it down, overwhelms it's scaling ability completely. Thereby denying service to the system to legitimate users, hence Distributed denial-of-service