An epiphenomenon is a secondary phenomenon that occurs alongside a primary phenomenon.

Often, a causal relationship between the phenomena is implied: the epiphenomenon is a consequence of the primary phenomenon. In medicine, this relationship is typically not implied: an epiphenomenon may occur independently, and is merely called an epiphenomenon because it is not the primary phenomenon under study. (A side-effect is a specific kind of epiphenomenon that does occur as a consequence.)

In philosophy of mind, epiphenomenalism is the view that mental phenomena are caused by physical phenomena, and cannot cause anything themselves. It was probably first mentioned by T. H. Huxley in 1874.

References

Huxley, T. H. (1874). On the Hypothesis that Animals are Automata, and its History, The Fortnightly Review, n.s.16:555-580. Reprinted in Method and Results: Essays by Thomas H. Huxley (New York: D. Appleton and Company, 1898).