A subatomic particle associated with the flow of time, whose presence usually signals temporal displacement and which has thrown several Starfleet crews into the past or the future.
Chronitons are particles that carry temporal properties, and unusual concentrations of them tend to mean that something has moved through time. In the Deep Space Nine two-parter Past Tense, chroniton particles trapped in the Defiant's hull by its cloaking device interfere with a transport and send Sisko, Bashir and Dax to San Francisco in 2024.
The Krenim build weapons around the particle. In the Voyager episode Before and After, radiation from a Krenim chroniton torpedo lingers in Kes and later drives her backwards through her own life, and the same weapons batter the ship in Year of Hell.
In Shattered, the Doctor prepares a chroniton-infused serum that lets Chakotay cross between sections of Voyager stuck in different time periods.