Materialism is a form of philosophical monism which holds that matter is the fundamental substance in nature, and that all phenomena, including mental phenomena and consciousness, are the result of material interactions.
Materialism is typically considered to be closely related to physicalism; the view that all that exists is ultimately physical. Philosophical physicalism has evolved from materialism with the discoveries of the physical sciences to incorporate far more sophisticated notions of physicality than mere ordinary matter, such as: spacetime, physical energies and forces, dark matter, and so on. Thus the term "physicalism" is preferable over "materialism", while others use the terms as if they are synonymous.
In philosophy, physicalism is the ontological thesis that "everything is physical", that there is "nothing over and above" the physical, or that everything supervenes on the physical. Physicalism is a form of ontological monisma "one substance" view of the nature of reality as opposed to a "two-substance" (dualism) or "many-substance" (pluralism) view. Both the definition of physical and the meaning of physicalism have been debated.
Physicalism is closely related to materialism. Physicalism grew out of materialism with the success of the physical sciences in explaining observed phenomena. The terms are often used interchangeably, although they are sometimes distinguished, for example on the basis of physics describing more than just matter (including energy and physical law).
'Cos, after all, what do scientists do when they find something that isn't a solid or a liquid or a gas or a plasma or a liquid crystal or a Bose-Einstein condensate or ... etc, etc. Why, they say "We have discovered a new form of matter which is neither a solid nor a liquid nor a gas nor a plasma nor a liquid crystal nor a Bose-Einstein condensate nor ... etc, etc".
It seems that science operates under an assumption of materialism/physicalism. Thus, "if we can address the matter scientifically, then it must be material/physical".
And this is probably okay, since materialism is a philosophical position and not a scientific one. It's a philosophical position that seems to be taken as an assumption as part of doing science (like empiricism or logic). This is probably due to the nature of non-material things (like souls) being inaccessible to study and investigation by others.
So science seems to prefer the material and to label as material anything it can investigate. This might be circular, but sometimes that's how definitions go. As you say:
If not, is materialism actually a theory, or just an agreement about how to use words?