In philosophy and logic, contingency is the status of facts that are not logically necessarily true or false.

In philosophy and logic, people draw a distinction between

possibility: "If it happened, it must be possible" -- If an act happened, it must be a possible act. Possible statements are not necessarily false. A "possibility", such as a coincidence, is either a "contingency", or a "necessity" (but not both).
contingency: a contingent act is an act which "could have not happened". Each contingent acts is also a possible act, but not vice versa. A contingent statement is not necessarily false, but it is not necessarily true either.
necessity: a necessary act is an act which "could not have not happened. In other words, a necessary act inevitably must have happened. Each necessary act is also a possible act, but not vice versa. A necessary statement is a statement that is necessarily true, such as a tautology.
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.