In the context of the law, an "act of God" is an accident caused by extraordinary natural forces. An accident caused by lightning, a flood, a hurricane, or an earthquake may constitute an act of God.
An act of God may be a defense to a personal injury action. For example, a tour company organizes backpacking expeditions in the Grand Canyon. On one such expedition, a lightning storm injures the backpackers. The backpackers file a personal injury action against the tour company. The tour company may defend the action by claiming that the storm was an act of God. If the court determines that the storm was an act of God, then the tour company will not be liable for the backpackers' injuries. However, the court may determine that the tour company should have cancelled the expedition because the storm had been forecast for days prior to the accident. In that case, the tour company will be liable for the backpackers' injuries.