DoS
From Hackepedia
Denial of Service is the exhaustion of resources on a computer or network. Legitimate users will be interrupted from doing their regular tasks and often a loss of monetary resources are involved. Denial of Service can cause stocks to fall and make administrators nervous and run around like chickens with their head cut off.
In order to minimise the effects of a DoS a Firewall can be used as well as creating policies that limit resources to an entity.
See also Ddos.