Water vapor or water vapour (see spelling differences), also aqueous vapor, is the gas phase of water. Water vapor is one state of the water cycle within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice.

Under normal atmospheric conditions, water vapor is continuously generated by evaporation and removed by condensation. Water vapor is a greenhouse gas along with carbon dioxide and methane. Whenever a water molecule leaves a surface, it is said to have evaporated.

Each individual water molecule which transitions between a more associated (liquid) and a less associated (vapor/gas) state does so through the absorption or release of kinetic energy. The aggregate measurement of this kinetic energy transfer is defined as thermal energy and occurs only when there is differential in the temperature of the water molecules. Liquid water that becomes water vapor takes a parcel of heat with it, in a process called evaporative cooling.

The amount of water vapor in the air determines how fast each molecule will return back to the surface. When a net evaporation occurs, the body of water will under go a net cooling directly related to the loss of water. In the US, the National Weather Service measures the actual rate of evaporation from a standardized "pan" open water surface outdoors, at various locations nationwide.

Others do likewise around the world. The US data is collected and compiled into an annual evaporation map. The measurements range from under 30 to over 120 inches per year.

Formulas can be used for calculating the rate of evaporation from a water surface such as a swimming pool.

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