From AMS Glossary
A small ice crystal of hexagonal tabular form.
The distance across the hexagonal facet of the crystal may be as large as 1–2 mm and the ratio of thickness to diameter as small as 1/100. This crystal form usually grows by vapor deposition at temperatures between -8° and -25°C. At temperatures between -12° and -16°C, as crystals grow and attain a higher fall velocity, provided that the saturation ratio approaches water saturation, the corners sprout and form dendritic (treelike) side arms.