A property or quality of a thing which is not essential to it, as whiteness in paper; an attribute.

an essence, a characteristic, that helps define a thing

a terrible thing

Aristotle partitioned reality into substance and accident. An accident is an incidental property of a thing.

The logical fallacy of accident, also called destroying the exception or a dicto simpliciter ad dictum secundum quid, is a deductive fallacy occurring in statistical syllogisms (an argument based on a generalization) when an exception to the generalization is ignored. It is one of the thirteen fallacies originally identified by Aristotle. The fallacy occurs when one attempts to apply a general rule to an irrelevant situation.