An antineutrino was a type of subatomic particle, the antimatter counterpart to the neutrino.
The starship USS Voyager was bombarded by antineutrinos when Ensign Seska attempted to activate a Sikarian spatial trajector unit aboard the ship. The ensuing antineutrino bombardment of Voyager's plasma manifold dramatically raised the warp core's plasma temperature, risking a containment breach. (VOY: "Prime Factors")