Full Idea
An 'accident' [sumbebekos] is something which may possibly either belong or not belong to any one and the self-same thing, such as 'sitting posture' or 'whiteness'. This is the best definition, because it tells us the essential meaning of the term itself.
Gist of Idea
An 'accident' is something which may possibly either belong or not belong to a thing
Source
Aristotle (Topics [c.331 BCE], 102b07)
Book Reference
Aristotle: 'Posterior Analytics and Topica', ed/tr. Tredennick,H/Forster,ES [Harvard 1960], p.285
A Reaction
Thus a car could be red, or not red. Accidents are contingent. It does not follow that necessary properties are essential (see Idea 12262). There are accidents [sumbebekos], propria [idion] and essences [to ti en einai].
Related Idea
Idea 12262 An 'idion' belongs uniquely to a thing, but is not part of its essence [Aristotle]