"Materialism is the belief in the existence of objective reality."
Here is a small portion of the definition of "materialism," from _Dictionary of Philosophy_ (International Publishers, New York, 1984):
The scientific, philosophical trend opposed to idealism. There are two kinds of materialism (M):
(1) The spontaneous belief of all people in the objective existence of the external world.
(2) The philosophical world outlook which scientifically deepens and develops spontaneous M. Philosophical M maintains that the material is primary and the spiritual/ideal secondary. Implicit in this concept is the idea that the world is eternal, not created by "God," and is infinite in time and space. Maintaining that consciousness is a product of matter, M considers it the reflection of the external world, and therefore asserts the knowability of the world. In the history of philosophy, M was the world outlook of the progressive classes and strata of society, who were interested in correctly understanding the world, and in increasing humanity's power over nature. Accepting the achievements of science, M promotes the growth of scientific knowledge and the improvement of scientific methods. This acceptance of the facts revealed by science favorably influences humanity's practical activity and the development of the productive forces.
Medieval realism (MR):
A trend in medieval scholasticism maintaining that universal concepts possess real existence, and precede the existence of singular objects. MR continued Plato's line in the solution of the problem of the relation between the concept and the objective world, and between the universal and the singular.
Naive realism (NR):
A spontaneous materialist understanding of the world inherent in every person. The conviction that all objects exist independently of human consciousness. But NR is not a consistent, theoretically conceived scientific world outlook. A false interpretation of NR is given by subjective idealists (Berkeley, Mach and others).