What is it called when a solid turns directly into a gas

This process is called sublimation.

When heat is added to a solid, it turns into gas, the process is called sublimation.

For example, camphor, napthalene balls, etc.

Sublimation

Explanation:

Sublimation is the process of conversion of solids directly into vapor on the supply of heat. This occurs because the vapor pressure of the sublimate becomes higher than the atmospheric pressure and the melting has not yet happened because of insufficient heat. In other words, the substance cannot exist in its liquid form at the given temperature and the atmospheric pressure.

Example: Dry ice, camphor