Reductionism can mean either (a) an approach to understanding the nature of complex things by reducing them to the interactions of their parts, or to simpler or more fundamental things or (b) a philosophical position that a complex system is nothing but the sum of its parts, and that an account of it can be reduced to accounts of individual constituents. This can be said of objects, phenomena, explanations, theories, and meanings.
Reductionism strongly reflects a certain perspective on causality. In a reductionist framework, phenomena that can be explained completely in terms of relations between other more fundamental phenomena, are called epiphenomena. Often there is an implication that the epiphenomenon exerts no causal agency on the fundamental phenomena that explain it.
Reductionism does not preclude the existence of what might be called emergent phenomena, but it does imply the ability to understand those phenomena completely in terms of the processes from which they are composed. This reductionist understanding is very different from that usually implied by the term 'emergence', which typically intends that what emerges is more than the sum of the processes from which it emerges.
Religious reductionism generally attempts to explains religion by boiling it down to certain nonreligious causes. A few examples of reductionistic explanations for the presence of religion are: that religion can be reduced to humanity's conceptions of right and wrong, that religion is fundamentally a primitive attempt at controlling our environments, and that religion is a way to explain the existence of a physical world. Anthropologists Edward Burnett Tylor and James George Frazer employed some religious reductionist arguments. Sigmund Freud's idea that religion is nothing more than an illusion, or even a mental illness, and the Marxist view that religion is "the sigh of the oppressed," providing only "the illusory happiness of the people," are two other influential reductionist explanations of religion.
There is a certain degree of reductionism in the social sciences, which often try to explain whole areas of social activity as mere subfields of their own field. As an example, Marxist economists often try to explain politics as subordinated to economy, and sociologists sometimes see economy and politics as mere sub-spheres of society. Wikipedia, Reductionism