Act of God

In legal usage throughout the English–speaking world, an act of God is an event outside human control, such as sudden natural disasters, for which no one can be held responsible.

In the law of contracts, an act of God may be interpreted as an implied defense under the rule of impossibility or impracticability. If so, the promise is discharged because of unforeseen occurrences, which were unavoidable and would result in insurmountable delay, expense, or other material breach.

An example scenario could assume that an opera singer and a concert hall have a contract. The singer promises to appear and perform at a certain time on a certain date. The hall promises to have the stage and audio equipment ready for her. However, a tornado destroys the hall a month before the concert is to take place. Of course, the hall is not responsible for the tornado. It may be impossible for the hall to rebuild in time to keep its promise. On the other hand, it may be possible but extraordinarily expensive to reconstruct on such short notice. The hall would argue that the tornado was an act of God and excuses its nonperformance via impossibility or impracticability.

In other contracts, such as indemnification, an act of God may be no excuse, and in fact may be the central risk assumed by the promisor—e.g., flood insurance or crop insurance—the only variables being the timing and extent of the damage. In many cases, failure by way of ignoring obvious risks due to "natural phenomena" will not be sufficient to excuse performance of the obligation, even if the events are relatively rare: e.g., the year 2000 problem in computers. Under the Uniform Commercial Code, 2-615, failure to deliver goods sold may be excused by an "act of God" if the absence of such act was a "basic assumption" of the contract, and the act has made the delivery "commercially impracticable".

Recently, human activities have been claimed to be the root causes of some events until now considered natural disasters. In particular:

Such events are possibly threatening the legal status of acts of God and may establish liabilities where none existed until now.

...
Wikipedia