An accident was an unexpected and unplanned event, often resulting in harm, damage, or loss.
In 2373, when asked by Dulmur of the Department of Temporal Investigations why he had taken the USS Defiant back in time, Captain Benjamin Sisko told him that it had been an accident. His fellow agent Lucsly then asked if he wasn't contending that it was a predestination paradox and Sisko assured him that he wasn't. He told him that this was good and Dulmur agreed that they hated those. (DS9: "Trials and Tribble-ations")
After an accident at the liquid-hydrocarbon research base destroyed a third of the habitats on Titan, the surviving colonists sent the Titan ship to Earth with a message asking for help. The ship was fired upon and destroyed by Defense Force ships. (DIS: "People of Earth")