Accident is that which is present and absent without the destruction of its subject. It receives a two-fold division, for one kind of it is separable, but the other inseparable, e. g. to sleep is a separable accident, but to be black happens inseparably to a crow and an Ethiopian; we may possibly indeed conceive a white crow, and an Ethiopian casting his colour, without destruction of the subject.

They also define it thus; accident is that which may be present and not present to the same thing; also that which is neither genus, nor difference, nor species, nor property, yet is always inherent in a subject.