accident

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An unexpected and undesirable event, especially one resulting in damage or harm.
• noun An unforeseen event that is not the result of intention or has no apparent cause.
• noun An instance of involuntary urination or defecation.
• noun Lack of intention; chance.
• noun Philosophy A circumstance or attribute that is not essential to the nature of something.

from The Century Dictionary.

• noun In general, anything that happens or begins to be without design, or as an unforeseen effect; that which falls out by chance; a fortuitous event or circumstance.
• noun Specifically, an undesirable or unfortunate happening; an undesigned harm or injury; a casualty or mishap.
• noun The operation of chance; an undesigned contingency; a happening without intentional causation; chance; fortune: as, it was the result of accident; I was there by accident.
• noun That which exists or occurs abnormally; something unusual or phenomenal; an uncommon occurrence or appearance.
• noun Irregularity; unevenness; abruptness.
• noun An irregularity of surface; an undulation: as, the enemy was favored by the accidents of the ground.
• noun A non-essential.
• noun In grammar, a variation or inflection of a word, not essential to its primary signification, but marking a modification of its relation, as gender, number, and case. See accidence.
• noun Synonyms Chance, mischance, hap, mishap, fortune, misfortune, luck, bad luck, casualty, calamity, disaster.
• noun Property, Attribute, etc. See quality.

from the GNU version of the Collaborative International Dictionary of English.

• noun Literally, a befalling; an event that takes place without one's foresight or expectation; an undesigned, sudden, and unexpected event; chance; contingency; often, an undesigned and unforeseen occurrence of an afflictive or unfortunate character; a casualty; a mishap.
• noun (Gram.) A property attached to a word, but not essential to it, as gender, number, case.
• noun (Her.) A point or mark which may be retained or omitted in a coat of arms.
• noun A property or quality of a thing which is not essential to it, as whiteness in paper; an attribute.
• noun A quality or attribute in distinction from the substance, as sweetness, softness.
• noun Any accidental property, fact, or relation; an accidental or nonessential.
• noun obsolete Unusual appearance or effect.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An unexpected event with negative consequences occurring without the intention of the one suffering the consequences.
• noun Any chance event.
• noun uncountable Chance.
• noun transport, vehicles An unintended event such as a collision that causes damage or death.
• noun Any property, fact, or relation that is the result of chance or is nonessential.
• noun euphemistic An instance of incontinence.
• noun euphemistic An unintended pregnancy.
• noun philosophy, logic A quality or attribute in distinction from the substance, as sweetness, softness.
• noun grammar A property attached to a word, but not essential to it, as gender, number, case.
• noun geology An irregular surface feature with no apparent cause.
• noun heraldry A point or mark which may be retained or omitted in a coat of arms.
• noun law casus; such unforeseen, extraordinary, extraneous interference as is out of the range of ordinary calculation.
• noun military An unplanned event that results in injury (including death) or occupational illness to person(s) and/or damage to property, exclusive of injury and/or damage caused by action of an enemy or hostile force.
• noun uncountable, philosophy, uncommon Appearance, manifestation.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an unfortunate mishap; especially one causing damage or injury
• noun anything that happens suddenly or by chance without an apparent cause