Summa Theologica

A non-essential quality of a thing that individuates it. For example, whiteness is an accident belonging to an egg. Accidents inhere within a substance (see below). 

Actuality refers to what a thing is. Potentiality refers to what it is possible for a thing to be. A thing may hold many possibilities within it, which an agent can bring into actuality by acting upon it. For example, a match has the potentiality to be lit. It becomes actually lit by someone striking it.

God alone is pure act, with no potentiality. He causes things, and is not caused by anything. He is unchangeable and lacks nothing. 

An inclination toward something, from the Latin appetere, to strive after, to aim for. An important term in Aquinas’ moral theology and theory of the passions of man. The appetites are what drive us to pursue certain goals and make moral choices.