A high-energy state of matter. Very deadly...radioactive, hot, and toxic...very truly said by the first definition...however...it is not found only in Sci-fi...it is quite real, and is used in the real world. You never see it, because there is simply not enough funding yet to put this incredible technology in place.
Also, the military, like computers, is keeping plasma technology both as a weapon and as an energy source, out of civilian hands, at least for the next 40 years. The reason is to keep civilians from being more advanced than the military, and also to perfect this new technology.
The United States military is already preparing themselves with plasma armaments to be mounted probably first on vehicles such as fighter jets and then guns to be held in the hands of mobile infantry soldiers. The goal in mind is low cost, high-abundance ammunition.
As a power source, it is already being used at the central core of not only the International Space Station, but the central hubs of cities.
Believe it. Because if you don't, one day it WILL come back to haunt you. The United States already has a plasma weapons platform in orbit and on the ground, in sync with eachother, ready to fire on any location on Earth. The purpose of this weapons system is a more efficient way of erradicating a whole area without the nuclear fallout, splash damage, etc.
Get in The Loop. You'll find out stuff like this all the time, and will find it very fascinating and gratifying.