What is the name of the process by which a solid turns directly into a gas?

The process by which a solid turns directly into a gas is called sublimation.

Sublimation skips the liquid state entirely. It occurs when molecules at a solid’s surface gain enough energy to escape directly into the vapor phase, making it an endothermic physical change. The reverse process, in which a gas becomes a solid without becoming liquid, is deposition, also called desublimation.

Dry ice is the familiar example: solid carbon dioxide changes directly into gas at ordinary atmospheric pressure, rather than melting into liquid carbon dioxide. Ice and snow can also sublime slowly, especially when the surrounding water-vapor pressure is low. Freeze-drying uses this behavior under reduced pressure to remove ice from food or medicine.

Sublimation is often confused with evaporation, but evaporation begins with a liquid. Deposition explains frost forming on a cold surface, while condensation produces a liquid from a gas. Technically, many solids can sublime at extremely low rates; the term is most noticeable when the solid-to-gas transition is rapid enough to observe.