Plasma just means the substance is mostly charged particles rather than being mostly neutral. In practice, this means a lot of hydrogen and helium nuclei that have been separated from their electrons. We know where it is on the periodic table, it's just ionized.

This isn't too surprising. Most of the matter in the universe is in stars or dispersed in near vacuum, both of which are conducive to plasma formation.

Also, this means 99% of the visible ordinary matter in the universe.