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Transitions
Substances pass through the three states of matter at different temperatures. One type of transition is from solid to liquid, which occurs at the melting point of a substance. Another transition is from liquid to gas, which happens at the boiling point of that substance. When gases turn to liquids, this is known as condensation. A liquid turns to a solid at its freezing point.
Sublimation
Another type of transition is called sublimation. Sublimation is the act of a solid transforming directly into a gas without passing through a liquid phase. An example of sublimation occurs with "dry ice." Dry ice is carbon dioxide which, when exposed to room temperatures turns into a fog without ever becoming liquid. In nature, sublimation occurs when snow turns directly to water vapor without melting to water between. This typically occurs in low-humidity climates.
Triple Point
Sublimation occurs only when a substance is at a temperature and pressure below the "triple point." The triple point of a substance is the temperature and pressure at which the three states of matter are in equilibrium with one another.
Deposition
The opposite of sublimation is a process known as "deposition." Deposition refers to the process of a gas transitioning into a solid phase without the intervening liquid phase. An example of deposition in nature is the transformation of water vapor directly into ice or snow.