From The Art and Popular Culture Encyclopedia
In philosophy and logic, contingency is the status of facts that are not logically necessarily true or false. Contingency is opposed to necessity: a contingent act is an act which could have not been, an act which is not necessary (could not have not been). Contingency differs from possibility, in a formal sense, as the latter includes statements which are necessarily true as well as not necessarily false, while a statement cannot be said to be contingent if it is true necessarily.
In colloquial English, a contingency is something that can happen, but that generally is not anticipated. Planning for contingencies often requires a more imaginative approach, because contingencies are inherently not obvious. Large organizations, such as governments, are often criticized for not planning for contingencies because the construction of plans to deal with contingencies often involves thinking outside the box. Beforehand, contingencies are hard to predict; this failure to appreciate contingencies ahead of time has led to the formulation of Murphy's law.