In common usage, matter is anything that has both mass and volume.

In contrast to this view based upon mechanical properties of matter, a long-standing approach is the particulate theory of matter

The particulate theory of matter dates back to Leucippus (≈490 BC) and Democritus (≈470-380BC).

The history of the concept of matter is a history of the fundamental length scales used to define matter. Different definitions apply depending upon whether one defines matter on an atomic or elementary particle level. One may use a definition that matter is atoms, or that matter is hadrons, or that matter is leptons and quarks depending upon the scale at which one wishes to define matter.

The change in building blocks means that although matter still may be made up of atoms and molecules (because they are made from leptons and quarks), matter is more general than this, and can be made up of assemblies of leptons and quarks that are not atoms or molecules, such as a quark-gluon plasma, or nuclear matter.

Energy and mass are connected by the equation , which means mass can always be related to energy (see Mass–energy equivalence).

In this equation, the mass referred to is the relativistic mass, which includes kinetic energy of motion. See . The use of the concept "relativistic mass" has been the topic of much debate. See

See for example, , and .

Matter is commonly said to exist in four states (or phases): solid, liquid, gas and plasma. However, advances in experimental technique 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.

RHIC Scientists Serve Up "Perfect" Liquid

In physics and chemistry, matter and energy exhibit both wave-like and particle-like properties, the so-called wave-particle duality or matter wave. In this connection, physicists speak of matter fields, and speak of particles as "quantum excitations of a mode of the matter field".

In the realm of cosmology, extensions of the term matter are invoked to include dark matter and dark energy]], which are to explain some behaviors of the observable universe. They are not formed of the same building blocks that make up ordinary matter.

Common definition

[[image:DNA chemical structure.svg The common definition of matter is anything that has both mass and volume (occupies space).

The observation that matter occupies space goes back to antiquity. However, an explanation for why matter occupies space is recent, and is argued to be a result of the Pauli exclusion principle.