plasma, Electrically conducting medium in which there are roughly equal numbers of positively and negatively charged particles, produced when the atoms in a gas become ionized (see ionization). Plasma is sometimes called the fourth state of matter (the first three being solid, liquid, and gas). A plasma is unique in the way it interacts with itself, with electric and magnetic fields, and with its environment. It can be thought of as a collection of ions, electrons, neutral atoms and molecules, and photons in which some atoms are being ionized at the same time as electrons are recombining with other ions to form neutral particles, while photons are continuously being produced and absorbed. It is estimated that more than 99% of the matter in the universe exists in the plasma state.
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