Matter, generally is a substance (often a particle) that has rest mass and (usually) also volume. The volume is determined by the three-dimensional space it occupies, while the mass is defined by the usual ways that mass is measured. Matter is also a general term for the substance that makes up all observable physical objects.

All objects we see with the naked eye are composed of atoms. This atomic matter is in turn made up of interacting subatomic particles—usually a nucleus of protons and neutrons, and a cloud of orbiting electrons. Typically, science considers these composite particles matter because they have both rest mass and volume. By contrast, massless particles, such as photons, are not considered matter, because they have neither rest mass or volume. However, not all particles with rest mass have a classical volume, and fundamental particles such as quarks and leptons (sometimes equated with matter) are considered "point particles" with no effective size or volume. Nevertheless, quarks and leptons together make up "ordinary matter," and their interactions contribute to the effective volume of the composite particles that make up ordinary matter.

Matter commonly exists in four states (or phases): solid, liquid and gas, and plasma. . However, advances in experimental techniques have revealed other phases that were 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. 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).