Ionized gas that consists of electrons, ions and a few neutral particles. It is referred to as the fourth state of matter. Plasma was first described in 1879 by William Crookes. In space, almost everything is made from plasma, but Earth is an exception, plasma is almost non-existent here. Plasma is characterized by its temperature, density, and ratio of ions, electrons and neutral particles. Because it consists of charged particles that respond to electromagnetic fields and generate them themselves as they change their motion, plasma has a number of interesting properties. It is quasi-neutral, meaning that it contains approximately the same amount of positive as negative charges, so it appears electrically neutral. It exhibits collective behaviour, i.e. it responds to the external electromagnetic field as a whole.

In thermonuclear fusion research, the plasma over a hundred million degrees Kelvin hot is produced from hydrogen, because only under such conditions can fusion take place.