Plasma is one of the four states of matter, dealt with in plasma physics. It is a state born from when gases are heated and the particles become charged, becoming negative or positive ions. Most of what has been learned has been thanks to nuclear fusion, and the research has advanced fields ranging from computers, space physics, lighting and lasers.
"An urban legend regarding a weapon of mass destruction using plasma technology that has risen from conspiracy theories about the military-industrial complex. The weapon uses plasma created by irradiating electromagnetic waves and crossing them to attack potential targets. It can destroy and/or disarm enemy weapons and instruments, as well as cause mental illness or burn human targets. Giant balls of flames created from plasma experiments have been sighted, but the conspirators reported the sightings as UFO encounters. Plasma is thought to be the ultimate form of plasma weapon: one that has been fused with a demon."
—Shin Megami Tensei IV compendium
Plasma is a pallette swap of Chemtrail, obtainable in the DLC quest "Death Has Its Applications." A man found in the only room on the map offers his fusion data in exchange for 10-Point Card x10. After obtaining the data, Plasma can be summoned in the Cathedral of Shadows by fusing Flaemis, Mothman and Gremlin in a special fusion. It has the unique skills "Plasma Discharge" and "Megidoplasma" and can teach Flynn the Fog Breath skill through its Demon Whisper.