An Act of God, for insurance purposes is any loss caused by natural disaster where no person could have foreseen that event. Most losses are not acts of God a road traffic accident is full of human error and logically most driver at anytime could be expected to be involved in some sort of accident.
Tornado's, Earthquake, Tsunami, volcanic eruptions (in some cases) are possible Acts of God.
Insurance companies can and do sometimes refuse to pay on Act of God claims.
I believe several large insurers now refuse to cover many areas of the Gulf Coast against Hurricane damage, because of the large losses caused by Katrina in 2005. One usually needs to pay higher premioums to cover for Acts of God if one lives in regions where natural dangerous conditions apply. So Californians are not generally covered for earthquake damage and need to pay for such coverage in special insurance policies.