Occasionalism is a theory in philosophy that causality is essentially an illusion—that substances are created by God and that what appears to be cause and effect between substances is merely an instance of God directly causing certain actions in the world.
With respect to philosophy of mind, occasionalism is presented as an answer to one of the problems with the dualism of René Descartes. Descartes' theory is an interactionist one—that is, it describes mind and body as being causally linked. However, since Descartes asserts that the mind and body are separate entities, one being of physical substance and the other being of mental substance (and hence not being extended in space), an obvious question arises: how can the mind cause changes in the body, and how can changes in the body affect the mind?
Descartes never presented a good response to this question, but philosophers such as Nicolas Malebranche offered occasionalism as an answer: that the mind and body were connected by God's will, and that God directly caused changes in the physical world to correspond with the mind.
See: mind-body problem