This article is about the concept in the physical sciences. For other uses, see Matter (disambiguation).
Matter
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Matter is usually classified into three classical states, with plasma sometimes added as a fourth state. From top to bottom: quartz (solid), water (liquid), nitrogen dioxide (gas), and a plasma globe (plasma).
Before the 20th century, the term matter included ordinary matter composed of atoms, and excluded other energy phenomena such as light or sound. This concept of matter may be generalized from atoms to include any objects having mass even when at rest, but this is ambiguous because an object's mass can arise from its (possibly-massless) constituents' motion and interaction energies. Thus, matter does not have a universal definition, nor is it a fundamental concept, in physics today. Matter is also used loosely as a general term for the substance that makes up all observable physical objects.