Plasma is the fourth ionized state of matter, it's conformed by a quasi-neutral gas composed of charged and neutral particles, which exhibits a collective behavior; plasma is the most abundant form of matter in the universe (99%). They are profoundly influenced by the electrical interaction of the ions and electrons and by the presence of magnetic fields. Plasmas are classified by several parameters that include the amount of ionization, plasma density, and plasma temperature among others.
The Nobel prize-winning American chemist Irving Langmuir first used this term to describe an ionized gas in 1927: Langmuir was reminded of the way blood plasma carries red and white corpuscles by the way an electric fied fluid carries electrons and ions. Langmuir, along with his colleague Lewi Tonks, was investigating the Engineering and chemistry of tungsten-filament light bulbs, with a view to finding a way to greatly extend the lifetime of the filament (a goal which he eventually achieved). In the process, he developed the theory of plasma sheaths; the boundary layers which form between ionized plasmas and solid surfaces. He also discovered that certain regions of a plasma discharge tube exhibit periodic variations of the electron density, which we nowadays term Langmuir waves. This was the genesis of plasma Engineering. Interestingly enough, Langmuir's research nowadays forms the theoretical basis of most plasma processing techniques for fabricating integrated circuits.
In fact, a plasma is sometimes defined as a gas that is sufficiently ionized to exhibit plasma-like behavior. Note that plasma-like behavior ensues after a remarkably small fraction of the gas has undergone ionization. Thus, fractionally ionized gases exhibit most of the exotic phenomena characteristic of fully ionized gases. Plasmas resulting from ionization of neutral gases generally contain equal numbers of positive and negative charge carriers. In this situation, the oppositely charged fluids are strongly coupled, and tend to electrically neutralize one another on macroscopic length-scales. Such plasmas are termed quasi-neutral ("quasi'' because the small deviations from exact neutrality have important dynamical consequences for certain types of plasma mode). Strongly non-neutral plasmas, which may even contain charges of only one sign, occur primarily in laboratory experiments: their equilibrium depends on the existence of intense magnetic fields, about which the charged fluid rotates.