What exactly is materialism? Is it a philosophy, a scientific theory, or something else?
This request for a definition only is fine for Philosophy.
Ok, so which is it?
It is a brand of metaphysics that then grounds an approach to physics. And the distinction is important because metaphysics is about reasoned arguments about what might be the case (what "materialism", or anything else, might rationally mean), and then the science is the actual job of modelling the world employing the concepts.
So the metaphysics makes a reasoned guess at the ontology - the world as it actually might be. Then science uses it as epistemology - as a mental framework for modelling the world that indeed seems to work (and so it becomes dangerously tempting to call it "true").
Then there is the second question about what materialism actually is as a brand of metaphysics (and which parts of it does physics or science indeed employ and finds useful).
Definitions are easy to find. You could start with this Wiki one, and go from there.
The normal debate is of course about whether there is just the material realm and anything to do with form, purpose, mind, mathematics, etc, is somehow emergent or otherwise "after the fact". Or whether there is a second fundamental category of nature besides material causes.
So the word gains rational meaning by defining itself in opposition to what it is not. The usual mutally exclusive/jointly exhaustive approach to forming "right concepts" in metaphysics.
as far as i can tell, materialism is no different, as a philosophy, than physicalism or naturalism. you can look those up in Wikipedia, too. i think that philosophical materialism is inconsistent with any form of spiritualism or theism.
there is a more common use of the term "materialism" or "materialistic" to describe a value system that promotes affluence and the acquisition and consumption of money and material goods. i tend to think that most rich people are materialistic in this manner, but that's just my opinion.
Materialism historically predates what is now known as physicalism, but the two terms are used more or less interchangeably in many areas of discourse these days. Materialism was originally the idea that matter is the only thing there is, and has its origin in the ancient atomists. So construed the notion of materialism seems hopelessly out-dated, and these days physicalism (in a broad sense) is probably way more popular. Physicalism allows for non-material entities such as relativistic gravity, forces, energy and so on. In other words, physicalism is the idea that everything is physical (or supervenes on the physical).
I'd also like to point out that naturalism is very hard to characterize, and that it, depending on the details, can be consistent with non-materialistic and non-physicalistic theories.
All these concepts are tricky, and saying something like "I'm a materialist" isn't very informative at all; it's almost comparable to saying that you're a realist. (Realism, in the philosophical sense, is the idea that something is real. Saying that one is a materialist, realist or X-ist will just make those who have taken a philosophy class ask "about what?". And then there's the problems that subscribing to an ism just gives a pretty ill-defined base to build the rest of the theory upon, so unless you have some details, you might as well settle for saying "I believe that everything is, but I don't know what the domain of discourse is".)
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