Matter is a material substance that constitutes the observable universe and, together with energy, forms the basis of all objective phenomena.
The basic building blocks of matter are atoms. All matter shares certain fundamental properties. Every physical entity has gravitation, the property by which it attracts every other entity. Another inherent and permanent property of matter is inertia, which causes a body to resist any change in its condition of rest or its motion. The mass of a body is a measure of its inertia, though it is commonly taken as a measure of the amount of material contained in the body.
Matter in bulk may have several states, the most familiar of which are the gaseous, liquid, and solid states. Less clearly definable but also referred to as states of matter are plasma, clusters, and amorphous conditions such as the glassy state. Each such state exhibits properties that distinguish it from the others. Moreover, these general states can be subdivided into groups according to particular types of properties. Solids, for example, may be divided into metallic, ionic, covalent, or molecular based on the kinds of bonds that hold together the constituent atoms.
According to Albert Einstein's special theory of relativity, matter (as mass) and energy are equivalent. Accordingly, matter can be converted into energy and energy into matter. The transformation of matter into energy, for instance, results during nuclear fission, which involves the splitting of a nucleus of uranium or another heavy element into two fragments of almost equal mass.
Excerpt from the Encyclopedia Britannica without permission.