The philosophical position that everything which exists is ultimately physical, with no separate spiritual or immaterial realm standing behind or above the natural world. In political thought its significance is downstream: if human beings are wholly natural, then mind, morality and social order require natural explanations rather than divine ones.
The ancient atomists stated it first. Democritus proposed a world of atoms and void; Epicurus built an ethics on it; Lucretius' On the Nature of Things transmitted the whole system to the Renaissance and supplied the early modern period with a fully worked-out naturalism that owed nothing to Christianity. Hobbes was the most rigorous early modern materialist, extending the framework to sensation, appetite, and finally to sovereignty itself — a move that scandalized his contemporaries precisely because they saw where it led. The French Enlightenment pushed further, with La Mettrie's Man a Machine (1747) and d'Holbach's frank atheism.
Marx and Engels then used the word for something different, and the ambiguity persists. Historical materialism is a thesis about the primacy of production in explaining social change, not a metaphysics of matter, and much confusion in political argument follows from treating the two as one position.
Contemporary philosophy generally works within a broadly physicalist framework, as do the cognitive sciences, though the terms have shifted from matter to whatever physics turns out to countenance. The live objection is consciousness: critics argue that no account of physical process explains why there is something it is like to undergo it, and materialists divide between those who expect the gap to close, those who think the question is confused, and those who accept it as a genuine limit.