Plasma, often referred to as the fourth state of matter, is a unique and dynamic state in which gas is ionized, allowing free electrons and ions to coexist. Unlike solids, liquids, and gases, plasma exhibits collective behavior due to long-range electromagnetic interactions among its charged particles. It is the most abundant form of matter in the universe, making up stars, the interstellar medium, and even artificial applications on Earth.

The concept of plasma dates back to the late 19th and early 20th centuries when scientists first began studying ionized gases. The term "plasma" was coined in 1928 by Irving Langmuir while researching electrical discharges in gases. Since then, plasma physics has evolved into a vast field, influencing multiple scientific and technological disciplines, including astrophysics, nuclear fusion, space exploration, and industrial applications.

One of the defining characteristics of plasma is its ability to conduct electricity and respond to magnetic fields. Unlike neutral gases, where collisions dominate particle interactions, plasmas are governed by electromagnetic forces, leading to complex behaviors such as waves, instabilities, and turbulence. This property allows plasmas to sustain structures like magnetospheres around planets and controlled fusion reactors designed for energy production.