The Chroniton Torpedo is a specialised weapon used by a Delta Quadrant race known as the Krenim Empire against the Starship Voyager during an alternative timeline known as the Year of Hell.
These weapons go into a state of temporal flux when fired, allowing them to penetrate conventional shielding technology with apparent ease. Voyager suffered hits from many of these weapons, taking near catastrophic damage during the year.1
While in this time period during a visit to another version of this timeline, Kes was able to use a tricorder on an unexploded Chroniton torpedo to determine that it had a temporal flux of 1.47. This information was judged to be of critical importance in devising a form of shielding technology which would be effective against these weapons.