"Plasma is a medium composed of electrons and ions free to move in all spatial directions.  Because a charged particle can interact simultaneously with many other charged particles in its vicinity [...], plasma behaves as a collective medium and is macroscopically neutral."—Thank you, Michael Moisan & Jacques Pelletier, for "Physics of Collision Plasmas: Introduction to High-Frequency Discharges," in Physics of Collisional Sciences, Springer Netherlands, 2012, p.1