- a cloud of a class characterized by a composition of ice crystals and often by the production of halo phenomena and appearing as a whitish and usually somewhat fibrous veil, often covering the whole sky and sometimes so thin as to be hardly discernible: of high altitude, about 20,000–40,000 feet (6000–12,000 meters).
- a uniform layer of cloud above about 6000 metres (20 000 feet)
