Theory of how body and mind are related.
It can mean simply that there are two types of events, physical and mental, either of which can cause the other; for example, a pin-prick causes pain, and pain causes screaming.
More usually, though, it is a philosophical theory grounding interactions of this kind in the existence of two substances, body and mind (or soul), where physical events take place in the one and mental events in the other, while the two series of events can interact causally, as above. In this form its main representative is Rene Descartes (1596-1650), but the theory goes back, in less developed forms, at least as far as Plato (c.427-c.347 BC).
R Descartes, Meditations on the First Philosophy (1641)