Tempest

There's not much secret about Tempest anymore, if you're referring to the military study into Wim van Eck's work with electromagnetic radiation emminating from video displays. van Eck's work dates back to 1985. The idea is that with the right equipment you can "evesdrop" on someone's terminal by capturing the electromagnetic radiation and decyphering it. From what I understand, these days, special font's and window backgrounds can defeat Tempest (like the viewing window in the latest versions of PGP). Here's some more background info, if you'd like:

Is It Possible to Eavesdrop on Electromagnetic Radiation?
Electromagnetic Radiation from Video Display Units: An Eavesdropping Risk?
Soft Tempest: Hidden Data Transmission Using Electromagnetic Emanations