Plasma, in physics terminology, is one of the four fundamental states of matter, the others being solid, liquid, and gas. Yet unlike these three states of matter, plasma is not stable on Earth under ambient conditions. Plasma can be generated temporarily in lightening discharges. A very thin Plasma exist in the outer atmosphere.
Plasma is essentially, atoms broken up into their constituent positive ions and free electrons, otherwise known as ionized gas, with an overall charge of nearly zero.
In The Expanse occasionally torpedoes are described as using "Plasma Warheads". It is not explained though whether this is idiosyncratic naming or relates to their function.