Dialectical materialism is the economic, political and philosophical system of Karl Marx and Friedrich Engels combining traditional materialism with Hegelian dialectic. In brief, it is the philosophical theory that the only reality is matter.
The term dialectics comes from the Greek dialektik, the art of debate, especially to resolve differences rather than establish one of two views as true, systematically weighing – from a logical viewpoint and disregarding concepts outside the limits of experience – contradictory facts or ideas. Dialectic is the Socratic method of argument, and was referred to by Plato as the highest study. In ancient Greece, dialectics was the method used to arrive at truth; an element of truth could be revealed by the contradictions in the argument of an opponent and the challenger would overcome those contradictions using the dialectical method of thought. Dialectics later extended to the phenomena of nature. In the 19th century Fredrick Engels wrote about it in his Dialectics of Nature this way:
It is an eternal cycle in which matter moves, a cycle that certainly only completes its orbit in periods of time for which our terrestrial year is no adequate measure, a cycle in which the time of highest development, the time of organic life and still more that of the life of being conscious of nature and of themselves, is just as narrowly restricted as the space in which life and self-consciousness come into operation. A cycle in which every finite mode of existence of matter, whether it be sun or nebular vapour, single animal or genus of animals, chemical combination or dissociation, is equally transient, and wherein nothing is eternal but eternally changing, eternally moving matter and the laws according to which it moves and changes.
Dialectical materialism is a method of understanding reality using thoughts, emotions and the material world. This methodology is the combination of materialism and dialectics, which in its essence is the direct opposite of metaphysics.
Marx never used the term in his work, but the philosophy of dialectic materialism is one of the basic concepts of Marxism. For instance, the American Heritage Dictionary defines dialectical materialism as, "The Marxian interpretation of reality that views matter as the sole subject of change and all change as the product of a constant conflict between opposites arising from the internal contradictions inherent in all events, ideas, and movements." Wikipedia refers to it as "a strand of Marxism..." That assumption is controversial since the status of science and naturalism in Marx's thoughts were blurred. He talked about the materialist conception of history but Engels called it historical materialism.
The term dialectical materialism was actually coined by Joseph Dietzgen in 1887. Dietzgen was a socialist who corresponded with Marx during and after the failed German Revolution in 1848, and was responsible for constructing the concept of dialectical materialism. Marx, Lenin and Friedrich Engels play no part in its inception, though the term was mentioned in a biography of Engels by Karl Kautsky, also written in 1887. Georgi Plekhanov is responsible for introducing the term dialectical materialism to Marxist literature. Stalin further promoted the concept as a way to study society and its history, and called it Diamat. He enforced it as the doctrine of Marxism-Leninism.
Dialectical materialism follows the general Hegelian principle of philosophy of history, which is the development of the thesis into its antithesis, which is contradicted by reconciling their common truth and forming a new thesis. Hegel said truth was the product of history and that aspects of truth passed through several moments that include the moment of error. Negativity and error are part of the development of truth. Hegel's idealism said that history was a product of the spirit, but Marx believed that history is a product of the materialistic class struggle in society so the truth has its roots in the materiality of social existence.
Marx believed that matter precedes thought. Materialism is a realist philosophy of science. All phenomena in the universe consist of energy or matter in motion, and all matter is interconnected and interdependent, and develops according to the natural law that says the world exists outside and is independent of man's perception of it. Thought is just a reflection of the material world in the brain, and the world is thus, in principle, knowable. Marx turned Hegel idealism upside down and when he wrote Das Kapital:
The ideal is nothing else than the material world reflected by the human mind, and translated into forms of thought.
Dialectical materialists tend to associate primacy with class struggle. Marx's materialist philosophy brings out the point that philosophy must take a position in the class struggle. That position must be based on objective analysis of physical as well as social relations. If that doesn't happen the concept is reduced to spiritualist idealism, and that agrees with the philosophies of Kant and Hegel. Engels incorporated more of Hegel's thoughts into his laws of dialectics by reading Hegel's Science of Logic and he incorporates them in his work, Dialectics of Nature.
So between Engels's laws of dialectics and Lenin's laws of motion, which came from reading Hegel's work, the modern day dynamics of Marxism were born. In this reality, Marx only played a minor role in Dialectical Materialism.