note: the word Materialism is here used to indicate a particular philosophical position, rather than a hedonistic lifestyle.  A less well-knwon but more precise term is Physicalism.

Materialism is the simplest (or most simplistic) explanation of reality: the belief that all that exists is the physical; there are no higher realities; no psychic or spiritual truths independent of the physical world.  Materialism itself is a  meme, a specific, culturally determined way of thinking about reality.

A slightly longer definition: Philosophical materialism (physicalism) is the metaphysical view that there is only one substance in the universe and that substance is physical, empirical or material. Regarding the "big questions", the sceptical or Materialistic explanation of the universe is that everything is matter and energy, and there is nothing else.  Spiritual substance is a delusion. Consciousness is explained simply as an emergent phenomenon of the physical brain.  There can therefore be no such things as the "supernatural ", paranormal phenomena, post-mortem existence, or occult phenomena.  These are either delusions or reducible to physical forces.  Materialists are not necessarily atheists (as it is possible to identify God with the material universe, as in Pantheism).  However, Atheism is often a corollary of Materialism, especially in the sense of a denial of a supernatural personal  God or any sort of higher creative power.  Materialists do not deny the reality of such things as love or justice, beauty or goodness.

References and Links:

Materialism - Resources on Materialism from TheFreeDictionary.Com.

Philosophical Materialism - Richard C. Vitzthum - excellent coverage, brief historical overview, also covers the various schools or interpretations of Materialism.  Does not go into the most modern developments

Materialism entry in The Catholic Encyclopedia - long and detailed (if at times biased) description of the history of materialism up until the turn of the 20th century

See also Scepticism

From a biased Western perspective there are four historical stages in the development forms of Materialism (I hope at some point to get round to including here a coverage of non-western materialistic philosophies).

Materialism originated in ancient Greek philosophy during the 6th century b.c.e., and in China and India around the same time, if not even earlier.  In Greece in the 5th century the atomists  Leucippus and  Democritus argued that all that existst is matter (in the form of limitless number of tiny indivisible partcles - atoms) and empty space, and that the differences in the sense objects are due to variations in the size and shape of atoms and their combinations

The atomism of Democritus and Leucippus was strenuosly challanged by the great Greek philosophers Socrates, Plato, and Aristotle, although it found a supporter in  Epicurus in the 4th century BCE. Epicurus argued that ultimate reality consisted of invisible and indivisible bits of free-falling matter called atoms randomly colliding in the void. This was then taken up in the first century b.c.e. by the Roman philosopher and poet  Lucretius, who explained the atomic structure of the world in his masterful 7400-line philosophical poem De Rerum Natura, or, as it's usually translated, The Nature of Things (50 b.c.e.).   Following this, with the rise of the varuious mystery religions and especially of Christianity, materialism receeded as a philosophy of any influence in the West

During the long reign of Christianity, denial of spirit as the basic reality was condemned by the church (also, all other interpretations of the spirit were forbidden as well  ;-).  In the 17th century, modern philosophy emerged from the shackles of the church

The father of modern philosophy was the French philosopher  Rene Descartes.  Descartes wanted to arrive at a sure and certain system of knowledge, and in a series of famous "meditations" (essentially "thought experiments") he proposed a central and irreconcilable dualism of mind and body - the so called "Cartesian Dualism".  The relation of the mind or consciousness with the body has been one that philosophers have pondered over for centuries, but especially since Descarte's famous phenomenological thought experiments into the nature of consciousness in the 17th century.  Since then, resoving the mind-body problem has become a central conundrum of Western analytical philosophy

Descartes was not without his challangers.  In fact, inasmuch as Western philosophy of this time was (and still remains) basically exoteric rather than esoteric, there were basically three ways to understand the world and the nature of mind and body

1. mind and matter both exist and are able to interact by some mechanism (this is Descarte's position)
2. matter is a dream or collective creation of mind (subjective idealism)
3. mind is the by-product of matter, or is nothing but the product of matter (Materialism)

Subjective idealism has had it's occaisonal advocates, from  Bishop Berkeley (18th century) on, but it never made much of an impact in the critical, sceptical, physical-orientated West.  In the East it did however independently becomne a central philosophical school of Mahayana Buddhism, under the name of Yogachara, but that is another matter

In the 17th century West, however, materialism was revived by the scientist Pierre Gassendi and even more strongly by the political philosopher  Thomas Hobbes.  The 18th century the materialist emphasis was on human nature. It was insisted that mind and soul are dependent entirely upon the physical properties of matter.  The most influential writer of this time was Baron Paul d'Holbach, whose anonymously published La Systeme de la Nature ("The System of Nature"), appeared in France in 1770 and was promptly condemned by Louis XVI's government, which meant every copy had to be destroyed.  d'Holbach used the mechanical determinism on Newtonian physics and the "blank slate" (tabula rasa) of John Locke's psychology, to argue that all reality depends on the movement and distribution of matter. Every event in nature, including all human thought and moral action, is the result of an inexorable chain of causation rooted in the flux of atomic motion

Yet Cartesian Dualism remained the most popular, owing to it's compatibility with Christian and Deistic faith, but always suffered from the serious difficulty of explaining how two such very different entities as mind and matter could interact (Descartes suggested the pineal gland, a theme taken up by Theosophists and New Age thinkers who associate the pineal gland with the Ajna or the Sahasrara chakra of Indian Tantric thought)

For all this, there is no denying a very powerful and persuasive link between consciousness and the brain.  We now know taht emotions, memories, thoughts and so on can be aroused by electrostimulation of precisely defined brain regions by patients being kept awake and describing what they "see" or feel etc.  The dualistic mind-body theory hardly makes sense in this context.  Even take the case of ordinary sensation.  If I perceive something with my senses how is it that that impacts on my consciousness?

The rise of science and advances made in chemistry, physics, and mechanics contributed to the strengthening of materialism from the late 18th century through the early decades of the 20th.  The discovery of chemical elements led to the revival of atomic theory. The publication of Charles Darwin's works on evolution demonstrated the possibility that living organisms can be accounted for on a material basis without any need to refer to a biblical Creator or supernatural purposes. The invention of the computer in the later 20th century suggested to materialists that mind itself can be explained purely in terms of matter and of electrical connections within the brain tissue

During this time there developed many different schools of materialistic thought, and the world materialism itself came to have many different meanings, even in the purely philosophical; context.  Materialistic monism explains matter and mind as merely aspects of each other and derivatives of a neutral substance. Antireligious materialism is a rejection of the the theological dogmas of organized religion.

Dialectical Materialism was used by Karl Marx and Frederich Engels as the foundational methodology of their historic work The Communist Manifesto (and of all their works). The Manifesto, published in 1848, elucidated the principles and historical foundations of scientific socialism (termed "Marxism" today). In it they stated that in every historical epoch the prevailing economic system (mode of production) determines the character of social and political organization of the society. The political, social and economic order is reflected in society's culture, including religion, intellectual endeavors and art.

Each preceding order, through internal contradictions (posed by growth in productive forces and the rise of a rival class) is overthrown by that rival class, which brings into being a new political, economic and social order. A major example provided in the Manifesto is the overthrow of feudalism by the capitalist class, which consolidated the new economic order of the capitalist mode of production (manufactures), and created a new political and social order. It also brought into being a new rival class of its own, the working class.

Socialism, the political, economic and social order of the working class, will in like manner come into existence through the overthrow of capitalism by workers. Finally, the goal of Socialism is for it to result in Communism, a single world society without classes, state or scarcity.

(It should be noted, therefore, that Socialism and Communism are not the same, and that the Soviet form of "communism" was not related to the Marxist understanding of the term.).

Popular Materialism.  Most of all the prominance of science led to a sort of social scientistic Materialism, is largely a recent (mostly 20th century ) Western phenomenon, basically irrational, that has come about through substituting one dogma ( religion) for another dogma, Physicalism.  Because this latter is more sophisticated then the earlier forms, and claims authority from the discoveries of Western science (although science neither affirms nor denies non-physical realities), it is often presented as an alternative to religion.   The word "scientistic" can be used  instead of "scientific", because this form of materialism is not necessarily more scientific (in its reliance on scientific method, falsification, etc) than any other religion or philosophy.  Scientistic Materialism is usually associated with rationalism

Analytical Materialism
A lot of analytical philosophy revolves around the so-called mind-brain problem, the question of whether or not human consciousness is reducible in all respects to scientific laws. In the 1960s and 1970s there were a number of different schools of materialists who argued in different ways for the identity of the brain and the mind

Reductionist materialism, or identity theorists, claim that the mind is identical to the brain in all respects.  Mental states are identical to brain states, that facts about mentality are reducible to physical facts, i.e. facts about matter and material processes.  This sort of naive naturalism indicates that there is a little "picture" in our heads that perfectly reflects the external world.  This is essentially no different to the materialism of the Greek atomists, of Lucretius, Gassendi, Hobbes, D'Holbach, Marx and Lenin

Functionalism argues that mental states are functional states (rather than brain states) which connect input (environmental
stimuli), other mental states (interconnected functions), and output (behavioral responses) in a cognitive system by means of
causal relations.  Psychology therefore is the scientific study of these functional properties (while neuroscience is distinguishably the study of neural properties).  The most famous functionalist was pretty much demolished by was the English philosopher  Gilbert Ryle , famous for his hatchet job on Cartesian Dualism, who explained away the concept of mind as a "category mistake".   Ryle's arguments hardly seem tenable today (after all, who is the one experiencing the category mistake  ;-)  Nevertheless Ryle's evocative phrase "Ghost in the Machine" really illustrated the absurdity of a disembodied consciousness (a ghost) interacting with a solid material body

Trivia: Ryle's portent phrase was co-opted not only by Arthur Koestler, a staunch opponent of Behaviourism and champion of the holistic theory of consciousness, but also by the authors of a superb and quite philosophical Japanese cyberpunk anime of the same title

Behaviourism, now called Eliminative materialists, is even more extreme than functionalism.  Behaviourism claims that minds don't exist, that our 'vague talk' about things like feelings, thoughts, desires, etc, needs to be eliminated from our vocabulary and replaced with precise scientific terms referring only to brain states.   This brings to mind the Kashmir Shaivite observation that for a conscious being to deny his own consciousness is like a man using his tongue to speak to say "I have no tongue"

Property dualism said that although nonphysical substances or things (e.g., souls, vitalist 'life forces' etc) do not exist, but that there are nonphysical properties of physical matter.  Epiphenomenalism is a type of property dualism which contends that mental states are mere nonphysical by-products or effects of neural firings that themselves have no effects on the physical world, including the brain

Supervenience theorists say only that one's mental properties are wholly dependent on the features and characteristics of one's bodily processes (this is also the opinion of other physicalists, and indeed is the minimal physicalist position)

Modern Functionalists, Property Dualists, and Supervenience theorists all deny the naive realism of the identity theorists.  They all agree that human consciousness is irreducible to or inexplicable in terms of natural processes. They hold that subjective mental phenomena and experiences - called qualia - are unique to that person and incommunicable and unknowable to any one else. They moreover qualia and other such properties of consciousness could not be translated into the terms of physical science in any meaningful way and hence represented a reality not amenable to the laws of nature

The stage was set for the next, most recent phase of Materialism, which attempts to do precisely that!

For a while it seemed like studies in quantum physics, indeterminacy, subatomic particles and the varous paradoxes of the new physics demolished the materialist position once and for all.  In the 1970s Fritjof Capra wrote a seminal book, The Tao of Physics, pointing out (with not a few dubious points on either side) equivalences between the new physics and ancient Eastern philosophies.  This book became a New Age classic, and has exerted a tremendous impact on the popular alternative consciousness (less so on the mainstream)

Currently however the situation is changing.  New insights and modern interpretations of consciousness in terms of quantum physics, microtubules (tiny structures within cells), artificial intelligence, and cognitive science, have given rise to a new wave of consciousness theorists presenting materialistic explanations that are far more sophisticated than anything from analytical philosophy.  More than any of their predecessors, they have done pretty good work at explaining the physical emergence of consciousness.  For a small selection of links:

David Chalmers works mainly in the philosophy of mind, and in related areas of cognitive science and metaphysics

Stuart Hameroff has proposed the theory of quantum computing in brain microtubules as an intra-neural correlate of consciousness

Archives of QUANTUM-MIND@LISTSERV.ARIZONA.EDU - Quantum Approaches to Consciousness - a huge resource - check it out.

Modern quantum physical and philsophical analysis such as referred to above pretty much explains consciousness and its emergence from the brain far more complexly and far more completely than any other explanation.  Indeed, I myself might be a sort of quantum physical holistic materialist, were it not for the fact that materialism - even in it's Quantum Mechanics mode - fails so dismially to explain insights from parapsychology, Out of Body and Near Death experiences, the I Ching and other synchronistic phenomena, many occult and magickal phenomena, certain higher meditative and mystical experiences, and certain fortean phenomena.  In fact, the only way materialism can explain all these thinsg is to fall back on the tired old arguments of delusion, hallucination, coincidence, misinterpretation.... arguments that are facile in the extreme when compared with the sophistication of the new materialism

All of which proves, not that materialism is wrong, only that, as in the insightful parable of the blind men and the elephant, it pertains to one aspect of Reality, not the Totality.