Any ionized gas, that is, any gas containing ions and electrons.
ON-LINE GLOSSARY OF SOLAR-TERRESTRIAL TERMS of National Geophysics Data Center

A fourth state of matter (in addition to solid, liquid, and gas) that exists in space. In this state, atoms are positively charged and share space with free negatively charged electrons. Plasma can conduct electricity and interact strongly with electric and magnetic fields. The solar wind is actually hot plasma blowing from the sun. See magnetosphere.
NASA Earth Observatory