The process of conversion of a solid, directly to gas phase, is known as Sublimation.
A very common example is the conversion of camphor to vapor state or gas phase, when it is heated. Generally, when heated, substances will convert from solid phase to liquid phase and then to gas phase (on further heating). For example, if we heat ice (solid form of water), it will convert to water (the liquid form) and further heating will convert it to water vapors (gas phase). These processes are known as melting (solid to liquid conversion) and evaporation (liquid to gas phase). However, in case of sublimation, substances skip the conversion to liquid phase and directly convert to the gas phase. Another example of sublimation is conversion of dry ice (solid form of carbon dioxide) to carbon dioxide gas, at room temperature.
Hope this helps.