The playa and shore of Lake Hart, an endorheic desert lake in South Australia

An alkali flat (known in US and Mexico as a playa) is a dry lakebed, generally the shore of, or remnant of, an endorheic lake. Such flats consist of fine-grained sediments infused with alkali salts. Their surface is generally very dry, hard and smooth in the summer months, but wet and very soft in the winter months. While the playa itself will be devoid of vegetation, they are commonly ringed by shadscale, saltbrush and other salt-tolerant plants that provide critical winter fodder for livestock and other herbivores.

Many playas contain shallow lakes in the winter, especially during wet years. If the layer of water is thin and is moved around the playa by the wind, an exceedingly hard and smooth surface can develop. Thicker layers of water can result in a "cracked-mud" surface. Too little water can result in dune formation.

The Spanish name playa (which literally means "beach") is used most commonly in Mexico and the western US. The playas at Black Rock Desert in Nevada and the Bonneville Salt Flats in Utah are have both been used for setting land speed records. The Black Rock Desert is also home to the Burning Man festival.