denial-of-service attack

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A malicious attack on a network resource that prevents legitimate users from accessing the resource, typically implemented by initiating an overwhelming number of spurious requests for service.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Internet A type of network computer attack that attempts to render a particular service (e.g. web site) unavailable to its audience.