Chronitons are subatomic particles with temporal properties. Although not harmful to humanoids, chronitons can be fatal to lifeforms which exist outside of time, such as the beings known to the Bajorans as "Prophets," and their mortal enemies, the Pah-wraiths. (DS9: "The Assignment")
Chronitons are produced by the normal operation of Romulan cloaking devices, and are also oftentimes associated with the phenomenon of time travel. The Orb of Time was known to produce a surge of chronitons when in operation. (TNG: "The Next Phase"; DS9: "Past Tense, Part I", "Trials and Tribble-ations")
Overview
Deep Space 9 fires a chroniton beam at the Bajoran wormhole
In 2368, Lieutenant Commander Geordi La Forge and Ensign Ro Laren became "phased" due to exposure to chroniton particles being emitted from the cloaking device of a Romulan science vessel which the Enterprise was assisting. As the Romulans had attempted to produce an interphase generator by combining their cloaking device with a molecular phase inverter, this had the effect of altering La Forge and Ro's molecular structure, making it impossible for them to interact with anything but each other. (TNG: "The Next Phase")
In 2371, the chronitons produced by the USS Defiant's cloaking device, which had lodged in the ship's ablative armor matrix, shifted into a high state of temporal polarization due to a temporal surge from an exploding microscopic singularity passing through the Sol system. This caused a transporter beam passing through the polarized particles carrying Captain Benjamin Sisko, Lieutenant Jadzia Dax, and Doctor Julian Bashir, to be redirected through time, resulting in their materializing in San Francisco, the year 2024. Chief O'Brien and Major Kira were later able to use the remaining polarized chronitons to mount a rescue mission for their stranded crewmates. (DS9: "Past Tense, Part I", "Past Tense, Part II")
In 2373, a Pah-wraith inhabiting Chief O'Brien's wife Keiko forced Miles to perform a series of modifications to Deep Space 9's deflector grid, transforming the station into a massive chroniton array aimed directly at the Bajoran wormhole, where the temporal disruption would kill all of the Prophets. O'Brien was able to successfully complete the modifications, but instead of aiming the chroniton beam at the wormhole, he targeted the runabout which he and Keiko were aboard, driving the Pah-wraith out of Keiko's body. A year later, Kai Winn utilized a chroniton generator to drive out a Prophet and a Pah-Wraith who had inhabited the bodies of Major Kira and Jake Sisko, respectively. Although Captain Sisko insisted that the battle between the two, called "the Reckoning" in Bajoran mythology, be allowed to play out, Winn covertly activated the generator in an attempt to subvert Captain Sisko's role in Bajoran affairs. (DS9: "The Assignment", "The Reckoning")
After Molly O'Brien fell into a time portal on Golana, Jadzia Dax deduced from the chroniton signature that she had been sent around three hundred years into the past. (DS9: "Time's Orphan")
Krenim weapons were chroniton-based, allowing them to pass through enemy shields by being in a state of temporal flux. (VOY: "Year of Hell")
The Borg use chroniton fields throughout their vessels, projected through specially designed conduits. These fields are essential when Borg vessels travel through the Borg transwarp network, to keep the various parts of the cubes in temporal sync and minimize the temporal stresses on the vessels. The warp core of a Federation starship and the bio-neural circuitry can be reconfigured to create a similar effect, although temporarily. (VOY: "Shattered")