Sublimation of an element or compound is a transition from the solid to gas phase so rapidly that the liquid phase cannot be observed. Sublimation is a phase transition that occurs at temperatures and pressures below the triple point (see phase diagram).
At normal pressures, most chemical compounds and elements possess three different states at different temperatures. In these cases the transition from the solid to the gaseous state requires an intermediate liquid state. However, for some elements or substances at some pressures the material may transition directly from solid to the gaseous state. Note that the pressure referred to here is the vapor pressure of the substance, not the total pressure of the entire system.
The opposite of sublimation is deposition. The formation of frost is an example of meteorological deposition.

Sublimation purification
Sublimation is a technique used by chemists to purify compounds. Typically a solid is placed in a vessel which is then heated under vacuum. Under this reduced pressure the solid volatilizes and condenses as a purified compound on a cooled surface, leaving the non-volatile residue impurities behind. This cooled surface often takes the form of a cold finger. Once heating ceases and the vacuum is released, the sublimed compound can be collected from the cooled surface. Usually this is done using a sublimation apparatus‎.