The neuralizer was a mind-altering device developed in the mirror universe (2385) by Tabatha Brisk and Alexis Crimson, based on the work of the mirror universe's Simon Van Gelder. (Star Trek: New Empire)
Short for "neural neutralizer", this device used brain-wave manipulation to alter brain chemistry and stimulate paralyzing fear, rendering subjects extremely susceptible to conditioning and thus enabling mind control. Essentially, it "reprogrammed" a person. Stephen April used it to gain greater dominance over the crew of the ISS Arcadia and thereby prevent assassination attempts against him. It worked on most humanoids.