A luminous gel that Enterprise NX-01 crew members spread over their skin in the decontamination chamber after away missions, to neutralise alien microbes and spores under ultraviolet light.
Decon gel is part of the 22nd-century routine for returning from an unknown environment. Crew who may have been exposed to a pathogen strip to their underclothes in the decontamination chamber beside sickbay, bathe in ultraviolet light and rub the gel over themselves and each other while Doctor Phlox watches the readings from outside.
The procedure is introduced in the Enterprise pilot Broken Bow, when Trip Tucker and T'Pol have to treat one another for a protocystian spore picked up on Rigel X and argue about Vulcan attitudes to humans as they do so. It recurs in episodes including Sleeping Dogs, after a visit to a crippled Klingon ship, and Bounty.
Different formulations are kept for different contaminants, and Phlox also uses the chamber to isolate patients. By the era of the later series the biofilter built into the transporter has made the whole ritual unnecessary.