The idea the mind and body are separate entities and fundamentally distinct. It asserts that mental phenomena are non-physical and cannot be explained by physical brain events.
Two different types of dualism:
Substance Dualism
The mind and body are made of completely different and independent substances, as argued by Descarte in Meditations on First Philosophy - René Descartes.
Property Dualism
There is only one fundamental kind of substance (physical matter), but this substance possesses two distinct kinds of properties: physical properties and mental properties