Selgninaem

From Memory Alpha, the free Star Trek reference

Selgninaem is a substance that isn't normally picked up by the sensors of a Galaxy-class starship.

Furthermore, along with invidium, saltzgadum, lucovexitrin and jakmanite, it is one of only five undetectable substances that can exist in a nitrogen-oxygen atmosphere and can alter the molecular structure of glass.

Selgninaem is highly toxic and was thereby ruled out as having caused the malfunctions aboard the Enterprise at the end of 2366, as the whole crew would have died of poisoning. (TNG: "Hollow Pursuits")

Selgninaem is 'meaningles(s)' spelled backwards.