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Accident, as used in philosophy, is an attribute which may or may not belong to a subject, without affecting its essence. The word "accident" has been employed throughout the history of philosophy with several distinct meanings.

Aristotle made a distinction between the essential and accidental properties of a thing. For example, a chair can be made of wood or metal, but this is accidental to its being a chair: that is, it is still a chair regardless of the material from which it is made. To put this in technical terms, an accident is a property which has no necessary connection to the essence of the thing being described.

To take another example, all bachelors are unmarried: this is a necessary or essential property of what it means to be a bachelor. A particular bachelor may have brown hair, but this would be a property particular to that individual, and with respect to his bachelorhood it would be an accidental property. And this distinction is independent of experimental verification: even if for some reason all the unmarried men with non-brown hair were killed, and every single existent bachelor had brown hair, the property of having brown hair would still be accidental, since it would still be logically possible for a bachelor to have hair of another color.

The nine kinds of accidents according to Aristotle are quantity, quality, relation, habitus, time, location, situation (or position), action, and passion ("being acted on"). Together with "substance", these nine kinds of accidents constitute the ten fundamental categories of Aristotle's ontology.

Catholic theologians such as Thomas Aquinas have employed the Aristotelian concepts of substance and accident in articulating the theology of the Eucharist, particularly the transubstantiation of bread and wine into body and blood. According to this tradition, the accidents of the appearance of bread and wine do not change, but the substance changes from bread and wine to the Body and Blood of Christ.

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