From AMS Glossary
(Or plane dendrite; sometimes called stellar crystal.) An ice crystal exhibiting an elaborately branched (dendritic) structure of hexagonal symmetry, with its much larger dimension lying perpendicular to the principal (c axis) of the crystal.
Such crystals usually form by vapor deposition at atmospheric temperatures between about -14° and -18°C. In sufficient concentration they readily aggregate to snowflakes by interlocking of their branches.