From AMS Glossary
(Or dendrite.) A crystal, particularly a planar ice crystal, with its macroscopic form (crystal habit) characterized by intricate branching structures of a treelike nature.
Dendritic ice crystals possess hexagonal symmetry, and tend to develop when a crystal grows by vapor deposition at temperatures within a few degrees of -15°C, providing saturation is close to supercooled water. Similar forms occur by ice growth into supercooled liquid water at temperatures down to -10°C. Spatial dendrites grow in three dimensions from a central frozen drop.