A lake is a naturally occurring, relatively large body of water localized in a basin completely surrounded by dry land, with much slower-moving flow than any inflow or outflow streams that serve to feed or drain it. Lakes lie completely on land and are separate from the ocean, although, like the much larger oceans, they form part of the Earth's water cycle by serving as large standing pools of storage water. Most lakes are freshwater, but some are salt lakes with salinities even higher than that of seawater.

Lakes are typically much larger and deeper than ponds, which are also water-filled basins on land, although there are no official definitions or scientific criteria distinguishing the two. Most lakes are both fed and drained by creeks and rivers, but some lakes are endorheic without any outflow, while volcanic lakes are filled directly by precipitation runoffs and do not have any inflow streams. Lakes are also distinct from lagoons, which are shallow tidal pools dammed by sandbars at coastal regions.

Natural lakes are generally found in mountainous areas (i.e. alpine lakes), dormant volcanic craters, rift zones and areas with ongoing glaciation. Other lakes are found in depressed landforms or along the courses of mature rivers, where a river channel has widened over a basin formed by eroded floodplains and wetlands. Some parts of the world have many lakes formed by the chaotic drainage patterns left over from the last ice age. All lakes are temporary over long periods of time, as they will slowly fill in with sediments or spill out of the basin containing them.

Artificially controlled lakes are known as reservoirs, and are usually constructed for industrial or agricultural use, for hydroelectric power generation, for supplying domestic drinking water, for ecological or recreational purposes, or for other human activities.