A rare and potent mineral found in the food of the planet Platonius which, once absorbed through the pituitary gland, gives humanoids the power to move objects and people by thought.
Kironide occurs naturally in the plants and fruit of Platonius, the world settled by the Platonians in the episode Plato's Stepchildren. Over centuries of eating local food the settlers built up the mineral in their bodies, and their pituitary hormones turned it into psychokinetic power.
The dwarf Alexander, whose pituitary deficiency stops him absorbing it, is the only Platonian without the power and is treated as a servant and jester. When Parmen uses his abilities to torment Kirk, Spock and their crewmates, McCoy isolates kironide and injects a concentrated dose into Kirk and Spock.
Kirk's power soon exceeds Parmen's, forcing the Platonians to back down. Alexander refuses the injection, wanting nothing that would make him like his masters.