Frost is the solid deposition of water vapor

Water vapor

Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

 from saturated

Saturation (chemistry)

In chemistry, saturation has six different meanings, all based on reaching a maximum capacity...

 air. It is formed when solid surfaces are cooled to below the dew point

Dew point

The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...

 of the adjacent air as well as below the freezing point of water. Frost crystals' size differ depending on time and water vapour available. Frost is also usually translucent in appearance. There are many types of frost, such as radiation and window frost. Frost causes economic damage when it destroys plants or hanging fruits.
If a solid surface is chilled below the dew point

Dew point

The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...

 of the surrounding air and the surface itself is colder than freezing, frost will form on the surface.