Denial-of-service attack (DoS attack): an attack on a network that floods it with useless traffic thus blocking legitimate traffic from accessing resources. This can lead to slow response time from the target of the attack or can make it so the Internet site or service does not function at all.
Denial-of-service attacks are violations of the Internet Architecture Board’s (IAB) policy on Ethics and the Internet.
If you are noticing network degradation, you can check CITES Status of Service to see if anything is happening at that level of service.
For more information on DoS attacks, you can check:
Webopedia entry
Wikipedia article
