Firewalls

noun

1. A fireproof barrier used to prevent the spread of fire between or through buildings, structures, electrical substation transformers, or within an aircraft or vehicle.
2. The software that monitors traffic in and out of a private network or a personal computer and allows or blocks such traffic depending on its perceived threat.
3. A hypothetical phenomenon where an observer falling into a black hole encounters high-energy quanta at or near the event horizon.