What is the name of the process where a solid changes directly to a gas?

The process in which a solid changes directly into a gas is called sublimation.

Sublimation is a phase change that bypasses the liquid state. Dry ice, solid carbon dioxide, is a familiar example: at ordinary atmospheric pressure it turns directly into gas at about −78.5 °C. Solid iodine can also visibly sublime when heated, producing violet vapor. Ice and snow may sublime slowly when their vapor pressure exceeds the surrounding water-vapor pressure.

The reverse process is deposition, or desublimation, in which a gas becomes a solid without first becoming liquid. Frost forming from water vapor is a common example. Sublimation is an endothermic process because molecules must absorb energy to overcome attractive forces holding them in the solid structure.

Sublimation should not be confused with evaporation, which changes a liquid into a gas, or melting, which changes a solid into a liquid. It also describes a physical change, not a chemical reaction. Freeze-drying uses reduced pressure to make frozen water sublime, preserving foods and medicines.