Plasma (from Ancient Greek πλάσμα​, meaning 'moldable substance') is one of the four fundamental states of matter and was first described by chemist Irving Langmuir in the 1920s.

It consists of a gas of ions, atoms which have some of their orbital electrons removed, and free electrons. Plasma can be artificially generated by heating or subjecting a neutral gas to a strong electromagnetic field to the point where an ionized gaseous substance becomes increasingly electrically conductive. The resulting charged ions and electrons become influenced by long-range electromagnetic fields, making the plasma dynamics more sensitive to these fields than a neutral gas.

Plasma and ionized gases have properties and display behaviours unlike those of the other states, and the transition between them is mostly a matter of nomenclature and subject to interpretation. Based on the temperature and density of the environment that contains a plasma, partially ionized or fully ionized forms of plasma may be produced. Neon signs and lightning are examples of partially ionized plasmas.

The Earth's ionosphere is a plasma and the magnetosphere contains plasma in the Earth's surrounding space environment. The interior of the Sun is an example of fully ionized plasma, along with the solar corona and stars.

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