Potent pheromones given off by Orion women that make males of many species aggressive, suggestible and infatuated while giving females headaches, revealing that Orion men are in truth the slaves.
Orion pheromones are a biochemical weapon that the women of the species carry in their own bodies. Males exposed to them become possessive, quarrelsome and easy to command, females of other species suffer headaches, and over time a whole crew can be turned against itself.
They are revealed in the Enterprise episode Bound. The Orion trader Harrad-Sar presents Captain Archer with three slave women, Navaar, D'Nesh and Maras, who soon have the men of the NX-01 fighting one another. Only T'Pol, as a Vulcan, and Trip Tucker, protected by his psychic bond with her, stay clear-headed enough to stop the ship being delivered to the Orion Syndicate. Harrad-Sar confesses that among Orions it is the men who serve.
Later series, Lower Decks among them, return to the pheromones in a lighter vein.