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.
References
Material fact Wikipedia(Text) CC BY-SA