Short for
Processor Serial
Number.
Brilliant idea implemented by
Intel in their
Pentium !!! processor to
spy on their
customers.
Intel pissed off many privacy groups with this stunt, so they made a feature to disable the PSN. That didn't last long, because a brilliant hacker found out how to remotely re-enable the PSN and force a reboot, demonstrating the weakness in the feature.
Intel originally claimed the serial number was to protect consumers from purchasing fraudulantly overclocked processors, and to make online transactions more secure.