plasma is the fourth state of matter — a gas so hot that its atoms separate into free electrons and nuclei, making it electrically conductive and responsive to magnetic fields.
Beyond the physics sense, the word also names the fluid component of blood, and both senses trace to the Greek plasma, "something molded or formed." The physics usage was coined in 1928 by Irving Langmuir, who thought the ionized gas in his discharge tubes carried charged particles the way blood plasma carries cells. Plasma is by far the most common state of ordinary matter in the universe: stars, including the sun, are plasma, as are lightning channels and the glow inside a neon tube.
What separates a plasma from a merely hot gas is ionization. Once electrons are stripped from their nuclei, the resulting soup of charged particles conducts electricity and — decisively for fusion energy — can be pushed, shaped, and confined by magnetic fields. This is the property the 📝Tokamak exploits: no material wall can touch a gas at 100 million degrees, but a magnetic field can hold one. Fusion plasmas must be kept hot, dense, and confined long enough for nuclei to collide and fuse, which makes plasma physics — its turbulence, instabilities, and transport — the central science of magnetic confinement fusion.