From AMS Glossary
(Also called obscuration.) Any collection of particles, aloft or in contact with the earth's surface, dense enough to be discernible to the observer.
Examples are haze, dust, smoke, fog or ice fog, spray or mist, drifting or blowing snow, duststorms or sandstorms, dust whirls or sand whirls, and volcanic ash. Potentially, all hydrometeors and lithometeors may be obscuring phenomena.