To capture, alter, and retransmit a communication stream in a way that misleads the recipient. As used by hackers, refers especially to altering TCP/IP packet source addresses or other packet-header data in order to masquerade as a trusted machine. This term has become very widespread and is borderline techspeak. Interestingly, it was already in use in its modern sense more than a century ago among Victorian telegraphers; it shows up in Kipling.