A solid object is in the states of matter characterized by resistance to deformation and changes of volume. In other words, it has high values both of Young's modulus and of shear modulus; this contrasts e.g. with a liquid, which has a low shear modulus. At the microscopic scale, a solid has these properties :

• The atoms or molecules that compose the solid are packed closely together.
• These constituent elements have fixed positions in space relative to each other. This accounts for the solid's rigidity. In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. A crystal structure is composed of a unit cell, a set of atoms arranged in a particular way; which is periodically repeated in three dimensions on a lattice. The spacing between unit cells in various directions is called its lattice parameters. The symmetry properties of the crystal are embodied in its space group. A crystal's structure and symmetry play a role in determining many of its properties, such as cleavage, electronic band structure, and optical properties.
  • If sufficient force is applied, either of these properties can be disrupted, causing permanent deformation.
• Because solids have thermal energy, their atoms vibrate. However, this movement is very small, and cannot be observed or detected under ordinary circumstances. In a solid, the particles are vibrating in place. A solid doesn't flow.

The branch of physics that deals with solids is called solid-state physics, and 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.

The lightest known solids are aerogels. The lightest aerogel produced has a density of 1.9 mg/cm³ or 1.9 kg/m³ (1/530 the density of water).