Materialism is the philosophical position that the material world — that is, the "natural world" of physical matter — is the only thing that truly exists. It rejects supernatural explanations of observable phenomena, as well as philsophical idealism and spiritualism.
Materialism is the basis of all scientific inquiry, although many scientists do hold supernatural or spiritual beliefs that may even play a role in their scientific endeavors.
In common parlance, materialism also means the attitude of valuing material possessions over intangible things like love and honor. For this reason, the word can have a pejorative connotation.