A solid is a state of matter, characterized by a definite volume and a definite shape (i.e. it resists deformation). Within a solid, atoms/molecules are relatively close together, or "rigid"; however, this does not prevent the solid from becoming deformed or compressed. In the solid phase of matter, atoms have a fixed spatial ordering; because all matter has some kinetic energy, the atoms in even the most rigid solid move slightly, but this movement is "invisible".

Physicists call the study of solids solid state physics. This includes semiconductors and superconductivity. Solid state physics is a type of condensed matter physics.

Materials science is primarily concerned with properties of solids such as strength and phase transformations. It overlaps strongly with solid state physics.

Solid state chemistry overlaps both of these fields, but is especially concerned with the synthesis of novel materials.

See also: gas, liquid, plasma

In geometry, a solid is a three-dimensional figure. See solid geometry.