||This article provides insufficient context for those unfamiliar with the subject. (October 2009)|
A Relay attack is a type of hacking technique related to man-in-the-middle and replay attacks, in which an attacker relays verbatim a message from the sender to a valid receiver of the message. The sender may or may not be aware of even sending the message to the attacker; if the sender is aware, it is likely under the impression that the attacker is the intended receiver of the message.
A typical relay attack is when a hacker follows a car owner with a relay that forwards the frequency of the alarm's remote control to the car, waiting for the car owner to accidentally press the open button during his jogging, if pressing a button is necessary (for security reasons, it should be necessary, but for convenience of the owner and the hacker, it is often not necessary).