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: it is in essence 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 or effect to the essence or substance of the thing being described.
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.
Through Transubstantiation, the accidents of the appearance of bread and wine do not change, but the entire substance changes from bread and wine to the Body, Blood, Soul, and Divinity of Christ.
Transubstantiation is the English version of the Latin word transsubstantiatio. It contains the following word elements:
** trans** - "to cross over, across, beyond, through, on the other side of, to go beyond"
substantia - translates Greek ousia meaning “that which is one’s own, one’s substance or property; the being, essence, or nature of anything.”
The Greek word for transubstantiation is metousiosis which means “a change of essence, inner reality.”
The word essence comes from the Latin word essentia, meaning “being, essence,” (to translate Greek ousia “being, essence”) from essent and esse which mean “to be.”