This is called sublimation -a solid changing into a gas without passing through the liquid phase.
Sublimation is a change of phase. Substances change between solid, liquid, or gas phases depending on the temperature and pressure.
This is a phase diagram. In this diagram solids are represented by the space above the red line, gases represented by the space below the blue line, and liquids represented by the space to the right of the green line.
The triple point is where a substance can exist in all three states at the same time in equal amounts.
Sublimation can be seen on this graph where the space for solids meets the space for gases (the red line below the triple point.)