Plasma was first identified (as "radiant mattter") by Sir William Crookes in 1879. Sir J.J. Thomson identified the nature of the matter in 1897. It was Irving Langmuir who assigned the term "plasma" in 1928.
It is odd to consider that plasma is actually the most common phase of matter, especially since it was the last one discovered. Flame, lightning, interstellar nebulae, stars, and even the empty vastness of space are all examples of the plasma state of matter.