a(n) is said to occur when hackers flood a network server or web server with many thousands of false communications or requests for services to crash the network.

A Denial-of-Service (DoS) attack

Explanation:

Hackers have many ways of interfering with or gaining access into a network or web server. One of the ways is to use Denial-Of-Service attack. In DoS attack, the network or web server is flooded with so many false communications or requests for services that the network might even crash.

The attack is might be so strong that it inundates the server with unnecessary queries so that even the legitimate requests from authorized/legitimate users might not be serviced or responded to.