An antichroniton is an elementary anti-particle associated with the flow of time in the opposite direction to which we are accustomed.
As such, it is associated with anti-time. It is the anti-particle equivalent of a chroniton. Chroniton particles are usually created by time travel or temporal fields. Bombarding such a chroniton field with antichronitons will usually neutralize and destroy the field.
When Kes was irradiated with chroniton radiation, in an alternate timeline, from a Krenim torpedo, she began moving through time. The Doctor successfully cured her by using an antichroniton field. (VOY: "Before and After")