A force majeure is a calamity that is beyond anyone's control and changes life in unpredictable ways. A tornado that devastates your town is a force majeure.
The noun force majeure is a French term that literally means "greater force" or "superior strength." It's no ordinary occurrence, but something that is overwhelming and unexpected, like a war, a hurricane, or an asteroid landing on your roof. It's also a legally valid excuse for not fulfilling an obligation or contract — everyone involved will agree that it was unavoidable.