(1) Faking the sending address of a transmission in order to gain illegal entry into a secure system. See
email spoofing,
phone number spoofing and
how to spoof your techie friends.
(2) Creating fake responses or signals in order to keep a session active and prevent timeouts. For example, mainframes continuously poll their terminals. If the lines to remote terminals are temporarily suspended because there is no traffic, a local device spoofs the host with "I'm still here" responses.