What is Material Fact?

Legal Definition
A material fact is a fact that would be to a reasonable person germane to the decision to be made as distinguished from an insignificant, trivial or unimportant detail. In other words, it is a fact which expression (concealment) would reasonably result in a different decision.

Falsification of a material fact in such a manner that, had the insurance company known the truth, it would not have insured the risk. Misrepresentation of a material fact gives an insurance company grounds to rescind a contract.

If material facts about the investment property lying in a floodplain had been communicated to the buyer, he would not have bought.
-- Wikipedia
Legal Definition
Crucial to the interpretation of a phenomenon or a subject matter, or to the determination of an issue at hand this is a specific type of confirmed or validated event, item of information, or state of affairs.
Legal Definition
(In contracts.) One which constitutes substantially the consideration of the contract, or without which it would not have been made. Lyons v. Stephens, 45 Ga. 143. (In pleading and practice.) One which is essential to the case, defense, application, etc., and without which it could not be supported. Adams v. Way, 32 Conn. 16S; Sandheger v. Hosey, 26 W. Va. 223; Davidson v. Hackett, 49 Wis. 186, 5 N. W. 459. (In insurance) A fact which increases the risk, or which, if disclosed, would have been a fair reason for demanding a higher premium; any fact the knowledge or ignorance of which would nafurally influence the insurer in making or refusing the contract, or in estimating the degree and character of the risk, or in fixing the rate. Boggs v. Insurance Co.., 30 Mo. 68; Clark v. Insurance Co.. 40 N. H. 338, 77 Am. Dec. 721; Murphy v. Insurance Co., 205 Pa. 444, 55 Atl. 19; Penn Mut. L. Ins. Co. v. Mechanics' Sav. Bank, 72 Fed. 413, 19 C. C. A. 286, 38 In R, A. 33.
-- Black's Law Dictionary