We know that ice, wax, and many other solids when heated first melt into liquid and then change to vapour. On heating, most of the solids first change into liquids and then into their gaseous states.
But, on heating, some solids change directly to their gaseous states without changing into liquids. Some examples of such solids are Nausadar (ammonium chloride), Camphor, Naphthalene and iodine. On cooling the gaseous state of such a substance, we get its pure form in a solid state.
The process in which a solid gets transformed directly into its gaseous state or a gas is transformed directly into its solid state, without changing into its liquid state, is known as sublimation.