At the link above you will come across a fairly good description of scientism, as the author understands it. You'll find others and not all will agree with him. I for instance don't. Not entirely and not as he does.
Scientism is one of those words who's meaning, who's description depends a lot on our experience in the subject of science. It depends a lot on how we see science and on how we were taught science to be. For my part I was taught to see science in a certain way, and to apply it in a certain way.
As I see it science isn't really a thing, but a way of discovering and learning. Science isn't really about knowledge, but more a way of learning. A way of discovery and how to verify and confirm that what you have learned is true. Science is a matter of faith, faith in our ability to verify and learn, but not faith in science per se, but in how the scientific method can and will lead to learning.
In short science is not a matter of fact, but more a matter in finding out. A matter or learning from observation, trial, and experience what is going on in the world. In a recent photo on the web we saw a raccoon sleeping in a man's living room. When a commenter said that raccoons are dangerous she was right. But at the same time she neglected to point out that sometimes some animals can be so comfortable around humans they really have no problem with them, and may well come to adopt a particular human as part of his family.
This is where science comes in, for by using science we are able to accept that things don't have to be the way we were told they should be. That there are things not dreamt of in our philosophy.
As I see it scientism is the belief that we can take the pronouncements of scientists on faith, that they are matters of faith and not to be discounted just because they are matters of revelation and authority. In scientism science consists of a series of hoary old prophets stomping down off a mountain bearing stone tablets bearing the Word of God upon them. As far as I can see scientism is science as religious belief, and that's not how science works.
And as far as I can see science and the scientific method can be applied to most anything, even things that we assume can't be, for as we learn of them we will learn just how they could work if they indeed were. For we are an animal that needs to understand, or to think we understand even when our understanding is wrong. For that gives us a basis on which we can learn of our errors and correct them. Science gives us the tools we need to discover and correct our mistakes. Scientism denies us those tools and insists that we take what we know as holy writ. Science is a pair of reading glasses making our world clearer. Scientism is gouging out the eyes because what you see, albeit oh so dimly and blurred, offends you.
So that is my understanding, how I see scientism. My understanding is necessarily blurred, incomplete, but at least it's a start and one I can improve upon. What is your understanding, can it be improved, and can you let it be improved?