The Fourth State of Matter
There are three classic states of matter: solid, liquid, and gas; however, plasma is considered by some scientists to be the fourth state of matter. The plasma state is not related to blood plasma, the most common usage of the word; rather, the term has been used in physics since the 1920s to represent an ionized gas. Space plasma physics became an important scientific discipline in the early 1950s with the discovery of the Van Allen radiation belts. Lightning is commonly seen as a form of plasma.
Matter changes state as it is exposed to different physical conditions. Ice is a solid with hydrogen (H2) and oxygen (O) molecules arranged in regular patterns, but if the ice melts, the H2O enters a new state: liquid water. As the water molecules are warmed, they separate further to form steam, which is a gas. In these classic states, the positive charge of each atomic nucleus equals the total charge of all the electrons orbiting around it so that the net charge is zero. Each entire atom is electrically neutral.
When more heat is applied, the steam may be ionized: an electron will gain enough energy to escape its atom. This atom is left one electron short and now has a net positive charge; now it is called an ion. In a sufficiently heated gas, ionization happens many times, creating clouds of free electrons and ions; however, not all the atoms are necessarily ionized, and some may remain completely intact with no net charge. This ionized gas mixture, consisting of ions, electrons, and neutral atoms, is called plasma.
NASA Marshall Space Flight Center