According to Purdue's Department of Chemistry, a solid is matter that retains a fixed volume and shape. Particles in a solid are rigid and locked in place.
Matter exists in three states: solid, liquid and gaseous. All three states of matter are formed of microscopic particles; the particles act differently in each state.
In gases, the microscopic particles vibrate and move freely at high speeds. They are well separated with no proper arrangement and flow easily, and the particles are free to move past one another. With so much space between particles, gases are highly compressible. The most typical example of matter in a gaseous state is the air in Earth's atmosphere.
In liquids, the microscopic particles are close together but still with no proper arrangement. The particles still vibrate — and flow easily — but are not as well separated as gases. Being denser than gas (having less free space between particles), liquids are not easily compressible. The most common example of matter in a liquid state is water.
In solids, the microscopic particles are firmly packed and usually in a regular pattern. Like gases and liquids, the particles in a solid vibrate; they are not, however, free to move and flow from place to place. Matter in a solid state is rigid with its particles locked into place. Like liquids, solids are not easily compressible (having even less free space between particles). A good example of a solid is a block of wood.