Whiteout, white-out, or milky weather is a weather condition in which the contours and landmarks in a snow-covered zone become almost indistinguishable. It could be also applied when visibility and contours are greatly reduced by sand. The horizon disappears from view while the sky and landscape appear featureless, leaving no points of visual reference by which to navigate; there is absence of shadows because the light arrives in equal measure from all possible directions. Whiteout has been defined as: "A condition of diffuse light when no shadows are cast, due to a continuous white cloud layer appearing to merge with the white snow surface. No surface irregularities of the snow are visible, but a dark object may be clearly seen. There is no visible horizon."

A whiteout may be due simply to extremely heavy snowfall rates as seen in lake effect conditions, or to other factors such as diffuse lighting from overcast clouds, mist or fog, or a background of snow. A person traveling in a true whiteout is at significant risk of becoming completely disoriented and losing their way, even in familiar surroundings. Motorists typically have to stop their cars where they are, as the road is impossible to see. Normal snowfalls and blizzards, where snow is falling at 3 or 5 centimeters per hour (1 or 2 in/h), or where the relief visibility is not clear yet having a clear field of view for over 9 meters (30 ft), are often incorrectly called whiteouts.