Sublimation
Sublimation is the phase change from solid directly to gas without passing through the liquid state, as when dry ice turns into carbon dioxide vapor.
Definition
Sublimation is the phase change from solid directly to gas, skipping the liquid state entirely, that happens when particles at a solid's surface gain enough energy to escape completely, and it happens more readily at low pressure for substances whose solid form has a high enough vapor pressure; the reverse process, gas turning directly to solid, is called deposition. Formally, sublimation occurs when a solid is held at a temperature and pressure below its triple point, where no liquid phase can be stable, so the solid-vapor equilibrium is reached directly, and its enthalpy equals the sum of the enthalpies of fusion and vaporization by Hess's law, with the sublimation curve on a phase diagram following the Clausius-Clapeyron relation using that larger combined enthalpy.
Example
Dry ice never leaves a puddle; it shrinks straight into a cold fog of carbon dioxide gas, sublimating at $-78.5^\circ\text{C}$ at normal air pressure, and snow can vanish on a freezing, sunny day without ever melting because the ice sublimates straight into dry air. Carbon dioxide's triple point sits at $-56.6^\circ\text{C}$ and $5.11\text{ atm}$, so at $1\text{ atm}$ liquid $CO_2$ simply cannot exist and the solid sublimes, absorbing $25.2\text{ kJ/mol}$, while water's triple point pressure is only $611\text{ Pa}$, which is why ice sublimates inside a freezer or a freeze dryer's near-vacuum but melts normally in a kitchen.
Key Insight
Freeze-dried food is made by freezing it and then lowering the pressure so the ice inside sublimates away, leaving the food's shape and nutrients intact at almost no weight. Sublimation also works as a purification method: gently heating an impure solid under vacuum lets the volatile compound sublime and redeposit as pure crystals on a cold surface while nonvolatile impurities stay behind, the same basic process by which comets shed their tails as solar heating sublimates their ice.