In the Philosophy of Mind, a Dualist theory of mind-body interaction which holds there is no causal interaction between minds and bodies, and each mental event is uniquely caused by a non-natural source (e.g., God) to occur in concert with a particular physical event.
Occasionalism, like Parallelism is an attempt to explain the correspondence between events in the mind (e.g., pain) and physical events (e.g., stubbing one’s toe) from a purely dualistic ontology. If minds are not made up of matter as brains are, then we must explain why activity in the mind is, or seems to be, conjuncted with events in the body. Being separate substances, minds and bodies cannot be interacting in a causal manner so there must be some other explanation for this phenomena.
A materialist would, of course, respond to this by claiming the constant conjunction of “mental” and physical events is good evidence to reject mind/body dualism and instead embrace the claim that all “mental” events just are physical events.
The most well know adherent of Occasionalism is Nicholas Malebranche.