Photoplasma was an energy signature.

In 2371, when the USS Voyager was distorted by a distortion ring, Chakotay was heading for the bridge on foot. To prevent getting lost, he retrieved a tricorder from a storage locker and rigged it to emit a photoplasmic trail. (VOY: "Twisted")

From the Star Trek Encyclopedia (4th ed., vol. 2, p. 141), "It may be a coincidence, but physicist Harvey P. Lynn, Jr., Star Trek's first scientific consultant, who worked with Gene Roddenberry during the early days of the original Star Trek series, suggested the term "photoplasma" for something in the first "The Cage". Although Roddenberry did not use this particular suggestion at the time, three decades later, Lynn's term finally made it to the screen."
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