Scientism usually means the acceptance of scientific theory and scientific methods as applicable in all fields of inquiry about the world, including morality, ethics, art, and religion.
Here, science is held to be the ultimate recourse in questions of public policy and even religion.
This viewpoint is typified by comments, such as 'Science demonstrates that it is useless (or useful) to use seatbelts in cars' or 'Science has shown that religion is wrong' or 'Science shows that capitalism (or communism or socialism) is correct.' In the case of such views as Marxism (and most types of totalitarian rationales) such views are also called historicism, relying on a 'scientific' analysis of inevitable historical patterns.
Scientism can mean the acceptance of scientific theory and scientific methods as applicable in all fields of inquiry about the physical, natural world. This definition is functionally equivalent to scientific naturalism.
Scientism can mean humanism and enlightenment values informed by science. In this context, scientism is "a scientific worldview that encompasses natural explanations for all phenomena, eschews supernatural and paranormal speculations, and embraces empiricism and reason as the twin pillars of a philosophy of life appropriate for an Age of Science." (Source: Michael Shermer, The Shamans of Scientism, Scientific American, 2002)
Scientism can be a pejorative term, attributing, for instance, a 'fetishization' of science to an individual. This accusation is potentially linguistically troublesome, because someone 'accused' of scientism could also indeed be a 'scientist', but this adjective, if used by the accuser, fails utterly as a label for those accused of scientism. What in fact should you call someone you accuse of scientism? A scientismist? Other 'crimes' to which the 'accusation of scientism' can be addressed include those exhibiting or proclaiming an ignorance (or denial) of a relationship/disjunction between metaphysical and natural phenomena. This sense of the term comes close to Hannah Arendt's use of it in The Origins of Totalitarianism; in her view, the Soviet Union and Nazi Germany had made the human condition a matter of scientific exactitude, and thus otherwise impossible moral or ethical questions (such as, "Can a man be worthless? And if so, can we euthanize him?") are easily resolved within the internally-consistent "scietific" methods of the state.
This viewpoint is typified by comments, such as 'there is one and only one method of science' or 'there is one and only one way to conduct valid scientific research' as well as by attempts to limit intellectual debate to the hegemony of the established position of the scientific community. Medical Scientism defines the term scientism in this sense of how valid medical research is supposed to be conducted.