there are different forms of materialism, and the one I use (dialectical materialism) is excentric. So this isn't a definition of all forms of materialism, only a marxist specific one known as dialectical materialism. Marxists tended to re-write things to suit logical consistency of their arguments and this is a specific response to the question of the relationship between thinking and being/mind-matter problem. it's worth treating it with some caution when applied in other contexts.
"the great basic question of all philosophy, especially of modern philosophy, is that concerning the relation of thinking and being... The answers which the philosophers have given to this question split them into two great campos. Those who asserted the primary of spirit to nature and therefore in the last instance assumed the world creation in some form or another... comprised the camp of idealism. The others, who regarded nature as primary, belong to the various schools of materialism." [F. Engels]
the reason it's dialectical is because nature is supposed to change itself and therefore does not require either human or divine consciousness to animate it. this was supposed to make it inherently atheist compared with other forms of materialism which could for example be pantheist if consciousness and matter were equated. Because nature is regarded as primary, it means that any explanation that regards spirit or consciousness as primary is rejected. It is therefore a philosophical worldview in marxist ideology that gets applied to everything rather than a single concept which is used selectively as a lens. This would include the rejecting concepts of god, the soul, supernatural entites as well less obvious ones such as free will. I'm not 100% sure how it works, but the form of materialism that is most nihilistic is eliminative materialism which denies the existence of consciousness. a distinction is that whereas eliminative materialism would such concepts as god are false, dialectical materialism would say there are an illusion arising from natural causes. man still has to create god based on logical inferrence. (I admit I'm unclear on the deatils as there are very few texts that clarify what they meant by "scientific atheism" in the USSR).