A weapon developed by the Krenim Imperium in the Delta Quadrant. When the U.S.S. Voyager encountered the Krenim in 2374, she was attacked and severely damaged by chroniton torpedoes. The ship was nearly destroyed by these devices as they ripped straight through Voyager's shields and impacted the hull. The damage was undone when Voyager destroyed the Krenim timeship and the wounds were erased.
Chroniton torpedoes exist outside the space-time continuum in a constant state of temporal flux, which was why they could penetrate the shields of a Federation starship. Voyager crewman Seven of Nine discovered that the temporal variance of one chroniton torpedoes was 1.47 microseconds. Whether the variance was specific to that torpedo or all of them had it is unknown. (VOY:"Year of Hell", "Year of Hell, Part II")