ice crystals forming a white deposit (especially on objects outside)

frost that grows outward from its substrate

A deposit of interlocking ice crystals formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches, plant stems, and leaf edges, wires, poles, etc. The deposition of hoarfrost is similar to the process by which dew is formed, except that the temperature of the frosted object must be below freezing.

a deposit of interlocking ice crystals (hoar crystals) formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches, plant stems and leaf edges, wires, poles, etc.; the surfaces of these objects are sufficiently cooled, mostly by nocturnal radiation, to cause the direct sublimation of the water vapor contained in the ambient air. (Top) A new layer of surface hoar on the snow. Note the quarter for scale. (Photo courtesy of K. Williams.) (Bottom) Russian translation prepared by Nina A. Zaitseva for the Arctic Climatology Project Arctic Meteorology and Climate Atlas.

Another name for frost. A deposit of hoarfrost occurs when air with a dew point below freezing is brought to saturation by cooling.