From AMS Glossary
Any characteristic external crystalline form.
For an ice crystal, may refer just to the ratio of the lengths of the crystal parallel to its c and a axes (at 90° to or parallel to the hexagonal basal plane), but usually it simply means the crystal shape, including dendritic, skeletal, prismatic, sectors, etc. The term is not applied to the internal crystal lattice. Slight variations in the growth rates of different crystal planes in a given crystal structure lead to quite different crystal habits. Such growth rate variations may result from variations in temperature and water vapor supersaturation of the environment in which the crystal grows.