Act of God is a legal term for events outside of human control, such as sudden floods or other natural disasters, for which no one can be held responsible. This does not protect those who put others in danger of acts of nature through negligence, such as an adult who instructs a group of children to stand under a tree to escape a lightning storm.
The legal term, "Act of God", does not necessarily imply that a God had a direct intervention which specifically caused a "natural" occurrence or disaster.
In the law of torts, an act of God may be asserted as a type of intervening cause, the lack of which would have avoided the cause or diminished the result of liability (e.g., but for the earthquake, the old, poorly constructed building would be standing). However, foreseeable results of unforeseeable causes may still raise liability. For example, a bolt of lightning strikes a ship carrying volatile compressed gas, resulting in the expected explosion. Liability may be found if the carrier did not use reasonable care to protect against sparks -- regardless of their origins. Similarly, strict liability could defeat a defense for an act of God where the defendant has created the conditions under which any accident would result in harm. For example, a long-haul truck driver takes a shortcut on a back road and the load is lost when the road is destroyed in an unforeseen flood. Other cases (and the preferred federal rule in the United States) find that a common carrier is not liable for the unforeseeable forces of nature. Memphis & Charlestown RR Co. v. Reeves, 1870, 77 U.S. 176.
A particularly interesting example is that of "rainmaker" Charles Hatfield who was hired in 1915 by the city of San Diego to fill the Morena reservoir to capacity with rainwater for $10,000. The region was soon flooded by heavy rains, nearly bursting the reservoir's dam, killing nearly 20 people, destroying 110 bridges (leaving 2), knocking out telephone and telegraph lines, and causing an estimated $3,500,000 in damage in total. When the city refused to pay him (he had forgotten to sign the contract), he sued the city. The floods were ruled an act of God, excluding him from liability but also from payment.