A computer that has been covertly taken over in order to perform some nefarious task. It is estimated that millions of PCs around the world have been compromised and, under the control of a third party, routinely transmit messages unbeknownst to the user.

Zombies are widely used to relay spam so that messages come from thousands of different IP addresses, avoiding detection and expanding volume at the same time.

Denial of Service
Before spam became such a popular pastime, zombies were used to transmit phony messages to a server in order to slow down service and disrupt the network. A "pulsing zombie" is one that sends bogus messages in periodic bursts rather than continuously, once again, to avoid detection. See spam, denial of service attack and botnet.