What is Act Of God?

Legal Definition
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.
-- Wikipedia
Legal Definition
At common law, an overwhelming event caused exclusively by natural forces whose effects could not possibly be prevented (e.g., flood, earthquake, tornado). In modern jurisdictions, "act of God" is often broadened by statute to include all natural phenomena whose effects could not be prevented by the exercise of reasonable care and foresight.
Legal Definition
In contracts. This phrase denotes those accidents which arise from physical causes, and which cannot be prevented.

2. Where the law casts a duty on a party, the performance shall be excused, if it be rendered impossible by the act of God; but where the party, byhis own contract, engages to do an act, it is deemed to be his own fault and folly that he did not thereby provide against contingencies, and exempt himself from responsibilities in certain events and in such case, (that is, in the instance of an absolute general contract the performance is not excused by an inevitable accident, or other contingency, although not foreseen by, nor within the control of the party. Chitty on Contr. 272, 8; Aleyn, 27, cited by Lawrence; J. in 8 T. R. 267; Com. Dig. Action upon the Case upon Assumpsit, G; 6 T. R. 650 ; 8 T. R. 259; 3 M. & S. 267 ; 7 Mass. 325; 13 Mass. 94; Co. Litt. 206; Com. Dig. Condition, D 1, L 13; 2 Bl. Com. 340; 1 T. R. 33; Jones on Bailm 104, 5 ; 1 Bouv. Inst. n. 1024.

3. Special bail are discharged when the defendant dies, Tidd, 243 ; actus Dei nemini facit injuriam being a maxim of law, applicable in such case; but if the defendant die after the return of the case and before it is filed, the bail are fixed. 6 T. R. 284; 6 Binn. 332, 338. It is, however, no ground for an exoneratur, that the defendant has become deranged since the suit was brought, and is confined in a hospital. 2 Wash. C. C. R. 464, 6 T. It. 133 Bos. & Pull. 362 Tidd, 184. Vide 8 Mass. Rep. 264; 3 Yeates, 37; 2 Dall. 317; 16 Mass. Rep. 218; Stra. 128; 1 Leigh's N, P. 508; 11 Pick. R. 41; 2 Verm. R. 92; 2 Watt's Rep. 443. See generally, Fortuitous Event; Perils of the Sea.
-- Bouviers Law Dictionary
Legal Definition
Inevitable accident; vis major. Any misadventure or casualty is said to be caused by the "act of God" when it happens by the direct, immediate, and exclusive operation of the forces of nature, uncontrolled or uninfluenced by the power of man and without human intervention, and is of such a character that it could not have been prevented or escaped from by any amount of foresight or prudence, or by any reasonable degree of care or diligence, or by the aid of any appliances which the situation of the party might reasonably require him to use.

Inevitable accident, or casualty; any accident produced by any physical cause which is irresistible, such as lightning, tempests, perils of the seas, an inundation, or earthquake; and also the sudden illness or death of persons. New Brunswick, etc., Transp Co., v. Tiers, 24 N, J. Law, 714, 64 Am. Dee, 394; Williams v. Grant, 1 Conn. 487, 7 Am. Dec. 235 ; Hays v. Kennedy, 41 Pa. 378, 80 Am. Dec. 627; Merritt v. Earle, 29 N. if. 115, 86 Am. Dec. 292; Story, Bailm. § 25; 2 Bl. Comm. 122 ; Broom, Max. 108.
-- Black's Law Dictionary
Legal Definition
An unusual force of nature, as an earthquake or a tornado. See 22 Am. St. Rep. 403; also 4 Ann. Cas. 128.
-- Ballentine's Law Dictionary