Sublimation is an endothermic process, which means that energy must be added to a substance to facilitate the process. Sublimation is the term that describes a solid substance turning directly into a gas.
Under a certain set of conditions, a substance can exist in both a solid, liquid and gas phase, called a triple point. By manipulating the temperature (increasing) or pressure (decreasing) of the substance near the triple point, it is possible to force it to change directly from a solid to a gas. In order to excite the atoms or molecules of a substance enough to turn into a gas, energy must be added, which makes sublimation an endothermic process.