A provision commonly found in contracts that frees both parties from obligation if an extraordinary event prevent one or both parties from performing. These events must be unforeseeable and unavoidable, and not the result of the defendant's actions, hence they are considered "an act of god".
A natural disaster (major earthquake, flood or hurricane) may be a force majeure if unpredictable
A rainstorm during an outdoor event is not - rain is a normal and common occurrence.
A war, or riot may be considered a force majeure.