Solid
A solid is a state of matter with a definite shape and volume because its particles are packed tightly and only vibrate in place.
Definition
A solid is a state of matter with a definite shape and definite volume, because strong attractive forces hold its particles in fixed positions where they can only vibrate rather than move away from their spots. Solids are nearly incompressible and are usually denser than the liquid form of the same substance, and crystalline solids arrange their particles in a regular repeating pattern while amorphous solids like glass and rubber do not. Bonding classifies solids further: ionic solids form a lattice of ions with high melting points and brittleness, metallic solids hold a cation lattice together in a sea of shared electrons, giving conductivity and malleability, network covalent solids link atoms in continuous covalent bonds for extreme hardness and high melting points, and molecular solids are discrete molecules held together only by weak intermolecular forces, giving them low melting points.
Example
A rock, an ice cube, and a wooden block all keep the same shape no matter what container holds them, because their particles are locked in place and merely vibrate. Table salt is a crystalline solid where sodium and chloride ions alternate in a cubic pattern, which is why salt grains form tiny cubes, and ice arranges water molecules into hexagons, which is why snowflakes have six sides; diamond, by contrast, is a network covalent solid where every carbon bonds tetrahedrally to four others, giving it a melting point above $3500^\circ\text{C}$ and the greatest hardness of any natural material, while molecular iodine, held only by weak dispersion forces, sublimes readily at room temperature.
Key Insight
Ice is a rare solid that floats on its own liquid, because water expands about $9\%$ as it freezes into a hexagonal crystal with more empty space than the liquid has. That same regular atomic spacing in a crystal happens to match the wavelength of X-rays, so crystals diffract them, and Bragg's law, $n\lambda = 2d\sin\theta$, turns that diffraction pattern into atomic positions, the method that revealed the structures of $NaCl$, DNA, and thousands of proteins.