A lake is an inland body of relatively motionless water that usually has a river or stream feeding into or draining out of it.

Lakes differ to lagoons and estuaries due to the fact they are not connected to the ocean. A lake is also larger and deeper than other inland water bodies such as ponds.

The study of inland water bodies and ecosystems is called Limnology.

A lake usually contains freshwater but some can be saltwater.

Each lake has a larger catchment area (drainage basin, watershed) which is a large encompassing land area where surface water from rain, snow/ice melt, or rivers converges into the lower lying lake.

Water in most lakes flows in and out via rivers and streams. Lakes that only lose water by evaporation or underground seepage are called endorheic lakes.

The lowest lake in the world is the Dead Sea that borders Israel and Jordan. The surface level of which is 418 metres (1,371 ft) below sea level. It is also one of the saltiest lakes in the world.

The highest lake in the world is the crater lake of Ojos del Salado at 6,390 m (20,965 ft) above sea level. The mountain lake sits on the border of Chile and Argentina.

The deepest lake in the world is Lake Baikal in Siberia, Russia. It is 1,637 m (5,371 ft) at its deepest point. Excluding the Caspian Sea it is the also the largest lake by volume in the world.

The longest lake in the world is Lake Tanganyika in Africa at 660 km (410 mi) it is also the 2nd largest by volume of water and the 2nd deepest.

The Caspian Sea has characteristics of being both a sea and a lake. The saltwater sea used to be connected to the world's oceans but became landlocked around 5.5 million years ago. It is often classed as a lake due to a lakes definition which would make it the largest lake in the world at 370,400 km² (143,244 mi²).

Located on the border of the United States and Canada are the Great Lakes of North America. They include 5 lakes: Michigan, Huran, Erie, Ontario, and Superior which together contain around 21% of the world's freshwater supply.

Lake Superior is the largest of the Great Lake and also has the largest surface area of any freshwater lake in the world at 82,000 km² (31,700 mi²).

Finland has the nickname "Land of the Thousand Lakes" as there are over 187,000 lakes in the country.

Most freshwater lakes on Earth are found in Northern areas of the Northern Hemisphere, Canada for example is estimated to have around 2 million lakes.

There are many natural processes that can form lakes. The advancement and retreat of glaciers over millions of years can leave behind bowl-shaped depressions which fill. Lakes can also form by tectonic related changes of the landscape, or by landslides that cause water blockages. Crater lakes and calderas are formed in volcanic craters. Oxbow lakes are small, crescent-shaped lakes created by the meandering of rivers over time.

A lot of lakes today are artificially made to generate hydro-electric power, for domestic water supply, for industrial or agricultural use, or for aesthetic and recreational purposes.

One of the lakes on Saturn's moon Titan called Kraken Mare is a massive 388,500 km² (150,000 mi²) making it larger than the Caspian Sea. The liquid is not water though (Titan's average temperature is -181 °C (-293.8 °F) so water would be frozen), instead it is a lake of liquid gas (methane and ethane).