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.

In the law of contracts, an act of God may be interpreted as an implied defence 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.