APA Dictionary of Psychology

in the doctrine of the four causes stated by Aristotle, the agent that initiates a causal process by bringing together the material cause (matter), the formal cause (form), and the final cause (purpose) of a particular effect. One of Aristotle’s examples of an efficient cause was a skilled craftsman who creates an artifact; another was a father who begets a child. More generally, the term is used to mean that which produces a given effect by a causal process.