The Latin term "force majeure" is defined as "a higher force, an irresistible force. An event which cannot be definitely foreseen or controlled." That's the definition from a law dictionary, anyway. A force majeure includes, but is not limited to, acts of God. Acts of God are a subset of force majeure. Force majeure is a class of circumstances beyond a party's control that make it impossible or commercially inadvisable to perform the contract at question. Like so many things in the law, there isn't a quick and easy definition. Whether the facts and circumstances surrounding each scenario amount to a force majeure has to be evaluated on a case-by-case basis. For example, for a national annual meeting starting on Tuesday, 9/11--the national emergency would surely be a force majeure. For an annual meeting starting the following week for a local group--probably not a force majeure.
If you are a planner and have event cancellation insurance, you may want to check your insurance policy to see what it defines as a force majeure, and whether cancellation for one of those reasons is excluded from the policy, as they often are.