A playa is a flat, dry, and vegetation-free area at the lowest part of an undrained desert basin. It is a location where ephemeral lakes form during wet periods and is underlain by stratified clay, silt, and sand, and commonly, soluble salts. Playas occur in intermountain basins throughout the arid southwestern United States. When flooded, a playa lake forms where fine-grained sediment and salts concentrate. Playas are among the flattest known landforms, and their slopes are generally less than 0.2 meters per kilometer. They require interior drainage to a zone where evaporation greatly exceeds inflow. Terminology is quite confused for playas because of many local names. A saline playa may be called a salt flat, salt marsh, salada, salar, salt pan, alkali flat, or salina. A salt-free playa may be termed a clay pan, hardpan, dry lake bed, or alkali flat. Playas are quite dynamic environments with surface channels, playa margins, sedimentary materials, and biota changing with each flooding event.