These words refer to what occurs either unexpectedly or without pre-arrangement or plan. Chance is the least formal of these. On one hand it can indicate coincidence: a chance meeting in the street. On the other, it can suggest an occurrence that is governed by no known physical laws: the development of quantum theory to explain seemingly chance events in the interaction of atomic particles. Accidental stresses the lack of intention or forethought, but is now strongly influenced by accident, meaning mishap, to suggest an error that brings undesirable or even disastrous results: an accidental misreading of her bank balance; the spectre of an accidental nuclear holocaust. Fortuitous can point to something that is apparently without cause or design, but it often suggests a good or desirable occurrence, an overtone that puts the word in the strongest possible contrast with accidental: a fortuitous change of plan that kept them out of the city during the accidental blackout.

Contingent can refer, most simply, to what is unexpected or unforeseen: a contingent thunderstorm that scattered the marchers. More often, the word points to something that is dependent on an uncertain event or condition; in this case the word is used with on or upon : an increase in your allowance that is contingent upon how well you do in your studies.

Incidental points to something occurring without design or regularity: an incidental shrub or two beside the path. Thus, the word can refer to something unplanned or unexpected, but usually of value, however sight, especially when this is a concomitant or side benefit gained in the pursuit of some other goal: an incidental knowledge of Montagnard folksongs gained during his service in Vietnam. Adventitious can be a much more formal substitute for incidental , pointing to a fortuitous acquisition or coming together: adventitious circumstances that encouraged the rise of capitalism. Like incidental , the word can also indicate something that is not inherent, particularly something that is extrinsic to a primary consideration: adventitious flaws that did not detract from the power of the time-battered piece of sculpture.