Dry Ice is the solid form of:

Sublimation is the direct conversion of a solid into a gas without passing through the liquid state, and only one everyday substance's solid form is popularly called “dry ice” because of this exact behaviour at atmospheric pressure.

Carbon dioxide (CO₂), when cooled and compressed into a solid block, sublimes at about -78.5°C directly into CO₂ gas without ever melting into a liquid; the escaping cold gas chills the surrounding moist air so that atmospheric water vapour condenses into the visible white fog. This solid, sublimating form of CO₂ is exactly what is called dry ice, used for refrigerated transport and stage-fog effects.

• Ammonia (NH₃) is a pungent gas used in fertilizers and refrigeration systems in its liquid form, but its frozen solid state is never referred to as dry ice.

• Nitrogen is cooled to a liquid (liquid nitrogen) for laboratory freezing, but nitrogen's solid form is likewise not called dry ice.

• Oxygen is essential for combustion and respiration and is stored as a gas or liquid, not as a solid known by the name dry ice.

Therefore, dry ice is the solid form of carbon dioxide.