An act of God is a natural catastrophe which no one can prevent such as an earthquake, a tidal wave, a volcanic eruption, a hurricane or a tornado. An act of God is generally considered an act attributable to nature without human interference. For example, damage from a tornado or a lightning strike would be considered an act of God. Damage would not be considered an act of God if it is caused by the property owner.
Acts of God have legal significance because "acts of God" are a legal excuse for delay or failure to fulfill an obligation or to complete a construction project. Many insurance policies don't cover damage caused by acts of God. At times disputes arise as to whether a violent storm or other disaster was an act of God (and therefore exempt from a claim) or a foreseeable natural event.
Whether an event is exempt from the terms of a contract depends on the language used in that particular contract and the interpretation of the event as being a foreseeable natural event or an act of God. In California, certain buildings are required to post warnings that the building is subject to structural damage in the case of events such as an earthquake. Local laws should be consulted for specific requirements.