Dictionary.com has the following definition of scientism:
"The belief that the assumptions, methods of research, etc., of the physical and biological sciences are equally appropriate and essential to all other disciplines, including the humanities and the social sciences."
I think this is a weak definition. Those with a scientistic mentality don't always think that the methods of the hard sciences are appropriate to the humanities. What they think is that the methods of the hard sciences are the only methods that can result in knowledge. Their conclusion is not that scientific methods are appropriate to the humanities, but that the humanities don't issue in knowledge because they cannot be pursued according to scientific methods.
I prefer the following definition of scientism, which may not be original with me, although I have not come across it in quite this formulation:
"Scientism is the mistake of taking the results of science to be more firmly known than its prerequisites."
It is, in other words, to think physics is more certain than metaphysics. It is to be confident that the atoms, electrons, quarks and black holes that result from scientific inquiry are "really real", but be suspicious of the microscopes, telescopes and centrifuges the scientist uses to deduce those electrons and black holes. This suspicion may even extend to the mind of the scientist who conducted the science.
Another way of saying it is that the scientistic mindset finds the everyday world of common experience to be more metaphysically suspect than the world constructed by scientific inquiry. It is to be more confident of the reality of bosons and protons than it is of cars, trees or the wind. The self-contradiction of scientism is that the possibility of science depends on the reality of the world of common experience; if the world of common experience is suspect, then the science that occurs in it is at least as suspect. And it is metaphysics that explores and defends the world of common experience.
At the origin of modern science, Galileo constructed a telescope and looked through it to discover the moons of Jupiter. Galileo's scientific discovery was only possible because he was here, the moons of Jupiter were there, and he was able to look from here to there through the telescope. Galileo did not discover the distinction between here and there; he brought the distinction into science and it is that distinction (among other things) that made his science possible. His science is a science of reality only because the distinction between here and there is a distinction not merely in our minds, but in reality as well. If the distinction between here and there is not real, or is just a fantasy of our minds, then the science conducted in light of it is a fantasy as well. In fact, Galileo's science is a science of reality only to the extent that the metaphysics supporting it is a metaphysics of reality.
The reader may recognize Immanuel Kant lurking in that last paragraph, and Kant is the great philosopher of modernity because he understood the meaning of the presumptions of modern thought and refused to turn from their consequences. He did not try to have his cake and eat it too, as so many modern thinkers do.
The scientistic mindset doesn't get this, and, unlike Kant, tends to think that science can produce the metaphysics that would underwrite its own possibility. This is endemic to contemporary mind science. It is amazing how many mind investigators quote Kant yet how little he is understood. Mind investigators find the metaphysical status of the mind to be dubious and mysterious, but have great confidence in the metaphysical ground of the scientific conclusions this shadowy mind draws. It's as though they think a fictional scientist in a movie can draw real conclusions about the size of the theater in which the movie is shown. Alas, a fictional scientist can only conduct a fictional science... and a science of reality must start in reality, and to do that it needs a metaphysics of reality.