Clarke County, VA Real Estate Lawyers, page 3

A provision in a contract stating that some or all of the terms of the contract will be altered or voided by the occurrence of a specific event. For example, a contingency in a contract for the purchase of a house might state that if the buyer does not approve the inspection report of the physical condition of the property, the buyer does not have to complete the purchase.

Principles of international law so fundamental that no nation may ignore them or attempt to contract out of them through treaties. For example, genocide and participating in a slave trade are thought to be jus cogens.

An extraordinary and unexpected natural event, such as a hurricane, tornado, earthquake or even the sudden death of a person. An act of God may be a defense against liability for injuries or damages. Under the law of contracts, an act of God often serves as a valid excuse if one of the parties to the contract is unable to fulfill his or her duties -- for instance, completing a construction project on time.

A doctrine that prevents a defendant from re-litigating an issue after it has been lost. For example, if your neighbor sues you for putting up a fence on his land and the court rules that your fence extends beyond your property line, you can't later file your own lawsuit seeking a declaration that the property line is incorrectly drawn.

The substitution of a new contract for an old one. A novation may change one of the parties to the contract or the duties that must be performed by the original parties.

A provision in a contract that preserves the rest of the contract if a portion of it is invalidated by a court. Without a severability clause, a decision by the court finding one part of the contract unenforceable would invalidate the entire document.