In British usage, a day on which the minimum temperature at the level of the ground or on the tops of low, close-growing vegetation falls to -0.9C (30.4F) or below (also called a "day with ground frost"). In more general usage, "frost" can refer to the condition that exists when the temperature of the Earth's surface and Earthbound objects fall below the melting point of water (0 degrees Celsius or 32 degrees Fahrenheit). Depending upon the actual values of ambient-air temperature, dewpoint, and the temperature attained by surface objects, frost may occur in a variety of forms. These include a general freeze, hoarfrost (or white frost), and dry freeze (or black frost). If a frost period is sufficiently severe to end the growing season (or delay its beginning), it is commonly referred to as a killing frost.

An environmental material which is primarily composed of minute ice crystals forming a fuzzy layer over a cold surface by direct deposition of water vapor.