The power supply used by lots of
telecommunications equipment. It stands for "
negative 48
volts
direct current".
-48V relative to what, you ask? Well, -48V relative to earth ground. Why not positive? Well, that's simple; telephone wires carrying a positive voltage relative to the earth will corrode in the ground due to electrolysis. This was a lesson learned early on in the history of telecommunications and has stayed with us.
This mystical voltage sticks around today simply because it is standardized. The voltage has been used since the early days of telecom, and all the infrastructure to support it is there. 48V is a good balance between being able to provide decent power without too much current, and not electrocuting anyone unfortunate enough to bridge it. Since modern packet-switched telecom equipment tends to share rack space with legacy circuit-switched systems, it is convenient for them to operate from the same voltage. Not only that, but since it is DC, it is easily supplied from batteries without having to use an inverter, so providing an uninterruptable power system is easier.
-48VDC distribution systems tend to use thicker wires and larger amp-capacity circuits than 120V and 240V AC mains, due to the fact that in order to consume the same amount of power from -48V, you need to handle more current. The systems can become unweildy, being able to handle thousands of amps and being fed by dozens of high capacity lead acid batteries. Everything better be well fused, or one short circuit can result in an impressive display. Vaporizing copper has the equivalent explosive force, pound-per-pound, as dynamite.