In the past I've made the distinction between science, true science, and scientism -- which is more like a religion.
The Skeptic's Dictionary does a better job of explaining than I can. I'd like to revisit this. Just don't have the energy at the moment. Work. Not complaining, but I'm busy and stressed and it's been like this for months.
Scientism, in the strong sense, is the self-annihilating view that only scientific claims are meaningful, which is not a scientific claim and hence, if true, not meaningful. Thus, scientism is either false or meaningless. This view seems to have been held by Ludwig Wittgenstein in his Tractatus Logico-philosophicus (1922) when he said such things as "The totality of true propositions is the whole of natural science..." He later repudiated this view.
In the weak sense, scientism is the view that the methods of the natural sciences should be applied to any subject matter. This view is summed up nicely by Michael Shermer:
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 (Shermer 2002).
On the other hand, the dictionary definition of 'scientism' is the attitude and method of the typical natural scientist, whoever that might be.