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If you came here looking for obsession with money and/or things, see consumerism.
Materialism is the philosophical stance that "all that exists", or is real, is material—that is, it consists of the various forms of matter and energy as we know them, and, possibly, other forms of "material" that we just simply don't know about yet. In modern parlance the word physicalism, which includes not only matter and energy but also such physical forces as wave/particle relationships and non-material forces produced by particles, is often preferred. The word materialism is usually used by creationists, and their use is usually vaguely defined.
Accusations of materialism in science tend to confuse two differing meanings of the word:
Ontological materialism is the belief, or assumption, that only material matter and energy exist. For the ontological materialist anything immaterial must be the product of the material. In principle all immaterial phenomena must be reducible to (explicable by) natural laws.
Methodological materialism is neither a belief nor an assumption but a restriction on method. Briefly stated it holds that a non-material assumption is not to be made. Science, for example, is necessarily methodologically materialist. Science wishes to describe and explain nature. Diversion into the "supernatural" begins to describe and explain matters that are not natural and obfuscate the natural.
Methodological materialism is a defining characteristic of science in the same way that "methodological woodism" is a defining characteristic of carpentry. Science seeks to construct natural explanations for natural phenomena in the same way that carpentry seeks to construct objects out of wood. In operating in this manner neither discipline denies the existence of supernatural forces or sheet plastics, their usefulness or validity. The use of either supernatural forces or sheet plastics is simply distinguished as belonging to separate disciplines.
Many scientists are also ontological materialists. Richard Dawkins espouses ontological materialism when he claims a completeness of science. Neil deGrasse Tyson, on the other hand, is a methodological materialist.
Both forms of materialism are very closely related to philosophical and methodological naturalism and at first glance seem almost identical. Materialism and naturalism differ only in that while naturalism assumes or studies the observable, materialism assumes or studies the observable and material.
 Confusion with consumerism
People often confuse materialism with consumerism, leading creationists and fundies to claim that all atheists are shallow and money-worshiping. This tendency is probably due to religions often presenting a dichotomy between the depraved material world and the supposed far better spiritual world.