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The word plasma has a Greek root which means to be formed or molded (the word plastic shares this root) and has a few definitions:

In physics and chemistry, plasma (also called an ionized gas) is an energetic state of matter in which some or all of the electrons in the outer atomic orbitals have become separated from the atom. The result is a collection of ions and electrons which are no longer bound to each other. This state of matter was first identified by Sir William Crookes in 1879, and dubbed "plasma" by Irving Langmuir.

The three lower-energy phases of matter are solid, liquid, and gas. Plasmas are the most common form of matter, comprising more than 99% of the visible universe. Commonly encountered forms of plasma include the Sun and other stars (which are plasmas heated by nuclear fusion), lit fluorescent lamps, lightning, the Aurora borealis, the solar wind, and interstellar nebulae. A plasma is also generated in front of a spacecraft's heat shield on reentering the atmosphere.

There are two broad categories of plasma, hot plasmas and cold plasmas. The Sun is an example of a hot plasma. Full ionization takes place, and the ions and the electrons are in thermal equilibrium. This is what would commonly be known as the "fourth-state of matter". A cold plasma is one where only a small fraction of the atoms in a gas are ionized, and the electrons reach a very high temperature, whereas the ions remain at the ambient temperature. These plasmas can be created by using a very high electric field to accelerate electrons which ionize the atoms. The electric field is either capacitively or inductively coupled into the gas. Common applications of cold plasmas include Plasma-Enhanced Chemical Vapor Deposition, Plasma Ion Doping, and Reactive Ion Etching.

The term plasma is generally reserved for a system of charged particles large enough to behave collectively. A microscopically small collection of charged particles is not usually called a plasma. The typical characteristics of a plasma are:

  1. Debye screening lengths that are short compared to the physical size of the plasma.
  2. Large number of particles within a sphere with a radius of the Debye length.
  3. Mean time between collisions usually are long when compared to the period of plasma oscillations.

See plasma physics for active research topics.

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