A solid is a state of matter where the atoms are tightly packed and held in place by strong interatomic forces. Solids have a definite shape and volume and are much more complex to compress than liquids or gases. As a result, most known materials are solids.

The three most common states of matter are solids, liquids, and gases. Of these, solids are the densest; their molecules (or atoms) are closest together on average. Solid substances have the highest melting and boiling points of all three phases—they require more heat energy to break apart than either liquids or gases. It also follows that a given substance will expand less when heated if it is already in the solid phase rather than either liquid or gas.