term:

Fortuitous Event

fortuitous event n

: an event of natural or human origin that could not have been reasonably foreseen or expected and is out of the control of the persons concerned (as parties to a contract)
: force majeure called also cas fortuit see also frustration compare inevitable accident

Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.