Accident

In Metaphysics, any property or quality of an object that is not necessary or essential to the thing itself.

For example, if my coffee mug is yellow, the color yellow is not essential to the mugs being a mug. It could have been blue, or white, or black, or any other color and maintained its essential nature. What is essential to the mug is its being ceramic (or some other non heat conducting material), being used to drink hot liquids, etc.