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Matter is anything that occupies space and has rest mass (or invariant mass). It is a general term for the substance of which all physical objects consist.[1][2] Typically, matter includes atoms and other particles which have mass. Mass is said by some to be the amount of matter in an object and volume is the amount of space occupied by an object, but this definition confuses mass and matter, which are not the same.[3] Different fields use the term in different and sometimes incompatible ways; there is no single agreed scientific meaning of the word "matter," even though the term "mass" is better-defined.

Contrary to the previous view that equates mass and matter, a major difficulty in defining matter consists in deciding what forms of energy (all of which have mass) are not matter. In general, massless particles such as photons and gluons are not considered forms of matter, even though when these particles are trapped in systems at rest, they contribute energy and mass to them. For example, almost 99% of the mass of ordinary atomic matter consists of mass associated with the energy contributed by the gluons and the kinetic energy of the quarks which make up nucleons. In this view, most of the mass of ordinary "matter" consists of mass which is not contributed by matter particles.

For much of the history of the natural sciences people have contemplated the exact nature of matter. The idea that matter was built of discrete building blocks, the so-called particulate theory of matter, was first put forward by the Greek philosophers Leucippus (~490 BC) and Democritus (~470–380 BC).[4] Over time an increasingly fine structure for matter was discovered: objects are made from molecules, molecules consist of atoms, which in turn consist of interacting subatomic particles like protons and electrons.[5][6]

Matter is commonly said to exist in four states (or phases): solid, liquid, gas and plasma. However, advances in experimental techniques have realized other phases, previously only theoretical constructs, such as Bose–Einstein condensates and fermionic condensates. A focus on an elementary-particle view of matter also leads to new phases of matter, such as the quark–gluon plasma.[7]