Actually scientism is very well defined by great philosophers of science like Popper and Putnam for instance.
It is, but as far as I am aware, 'scientism' is always used polemically by those who are critical of the universal applicability of the scientific method. No one who actually believes that the scientific method is the only means to knowledge ever refers to their position as 'scientism' The suffix 'ism' is usually applied to religion or similarly dogmatic ideologies, while scientists consider themselves to be open minded and dogma-free. To tag 'ism' to the end of science, implies that science is just as dogmatic as any religion. The way the word is put together is inherently biased against the position it is applied to. It's like calling a Catholic a Papist or telling a homosexual he is a sodomite. You may not mean it to be derogatory, but that's how it feels for those who get labeled with it.