Materialism is the philosophical belief that the world is made of material, and that there are no other types of entity (things). Everything is composed of material. Things that are not made of material, such as consciousness, are the result of actions by material. In other words, matter is the only real substance. Physicalism is a related term.
Especially, a materialist does not believe in gods or other supernatural entities. The materialist thinks the idea of a spirit is either meaningless, or not scientifically proven. Therefore, by definition, materialists are also atheists, though not all atheists are materialists.
Modern philosophical materialists extend the definition to include other basic entities proven by science. such as energy, forces, and the curvature of space. Matter and energy are known to be interchangeable, and much else, such as gravity, is caused by matter. However, the concept of matter is itself not entirely clear.
Materialism for Kids. Kiddle Encyclopedia.