The theory that consciousness is an accompaniment or by-product of neural processes, determined by them but exerting no influence upon them, so that it is but an "epiphenomenon" of matter, matter being the real phenomenon.

the view that all reality is a product of material causation. All substance is material in nature. The mind is the only exception. Although it is not composed of material, or, its intellect is not material substance, it is a consequent effect of material causation. Mind is not reduced to matter in this view of materialism.

The view that some events or states or properties are caused by the physical, but then do not interact with anything else physical. They have no causal powers themselves. So, for example, if the mind were an epiphenomenon, then it might exist because of something the brain does, but the mind cannot then affect the brain. This term is also used to refer to states or events or properties that do not causally interact with respect to some theory (though they might causally interact with something that isn't covered by the theory). So, in the latter use, the mind would be epiphenomenal with respect to psychology or neuroscience, say, if it isn't required to explain or understand our behavior.