act of God

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A violent or destructive natural event, such as a lightning strike or earthquake.
• noun Law An occurrence, such as a natural event, that is beyond human control and whose consequences are therefore not a basis for legal liability.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun law an unforeseen occurrence beyond one's control, such as a natural disaster