上述单词均有偶然和意外的含义。accidental主要指未曾预料的不幸事件或灾难，其无任何规律性，虽无故意或预谋，但后果影响常常却较严重[1]。chance为非正式用语，多指巧合或不属已知的自然法则调整的事件[2]。contingent经常表示无法预见或不可控制的紧急情况，如某些不可抗力等[3]。incidental多指附带发生的未曾计划或预料的事件，其多少都具有一些价值[4]。fortuitous指某事未经策划，其原因不明，在指事件时, 可表示该偶发事件非常不幸（以往多将该词误用作fortunate的同义词而认为其结局一般较好）[5]。casual指无规律，不正规， 无计划或不可预测，且常用于指附带发生的则无关紧要的小事等[6]。
 
[1 ] “ Employed in contradistinction to willful... produced by mere chance, or incapable of being traced to any cause. " Cf. Daphne Dukelow, The Dictionary of Canadian Law, at p. 6, Thomson Professional Publishing Canada (1991).
[2 ] “ Chance is least formal of these. On one hand it can indicate coincidence, on the other hand, it can suggest an occurrence that is governed by no known physical laws. ” Cf. The Editors of the Reader's Digest, Use the Right Word, at p. 77, The Reader's Digest Association Proprietary Ltd. (1971).
[3 ] “ Possible, but not assured;doubtful or uncertain;conditional upon the occurrence of some future event which is itself uncertain, or questionable. This term, when applied to a use, remainder, devise, bequest, or other legal right or interest, implies that no present interest exists, and that whether such interest or right ever will exist depends upon a future uncertain event. ” Cf. The Publisher's Editorial Staff, Black's Law Dictionary, abridged 6th edition, at p. 222, West Publishing Co. (1991).
[4 ] “ Depending upon or appertaining to something else as primary; something necessary, appertaining to, or depending upon another which is termed the principal ； something incidenta] to the main purpose. " Id. at p. 523.
[5 ] “ Occurring by chance. A fortuitous event may be highly unfortunate. Literally, the term is neutral, despite its common misuse as a synonym for fortunate. * Cf. Bryan A. Garner, Black's Law Dictionary, 7th edition, at p. 664, West Group (1999).
[6 ] " Occurring without regularity, occasional; impermanent, as employment for irregular periods. Happening or coming to pass without design and without being foreseen or expected； unforeseen, uncertain； unpremeditated. ” Cf. The Publisher’s Editorial Staff, Black's Law Dictionary, abridged 6th edition，at p. 150 , West Publishing Co. (1991).