Masking agent
A masking agent was a chemical that could hide the presence of another chemical.

In 2151, Jonathan Archer asked Dr. Phlox if he could create a chemical that would mask the  wraiths so that the Eska would have a harder time hunting them. Phlox did so, and Archer gave the masking agent to a wraith he had encountered, who distributed it to the others like her. ()
Chemical compounds