What is Lake?

Legal Definition
A lake is an area of variable size filled with water, localized in a basin, that is surrounded by land, apart from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, and therefore are distinct from lagoons, and are also larger and deeper than ponds, though there are no official or scientific definitions. Lakes can be contrasted with rivers or streams, which are usually flowing. Most lakes are fed and drained by rivers and streams.

Natural lakes are generally found in mountainous areas, rift zones, and areas with ongoing glaciation. Other lakes are found in endorheic basins or along the courses of mature rivers. In some parts of the world there are many lakes because of chaotic drainage patterns left over from the last Ice Age. All lakes are temporary over geologic time scales, as they will slowly fill in with sediments or spill out of the basin containing them.

Many lakes are artificial and are constructed for industrial or agricultural use, for hydro-electric power generation or domestic water supply, or for aesthetic or recreational purposes or even for other activities .
-- Wikipedia
Legal Definition
A large body of water, contained in a depression of the earth's surface, and supplied from the drainage of a more or less extended area. Webster. See Jones v. Lee, 77 Mich. 35, 48 N. W. 855; Ne-pee-nauk Club v. Wilson, 96 Wis. 290, 71 N. W. 661. The fact that there is a current from a higher to a lower level does not make that a river which would otherwise be a lake; and the fact that a river swells out into broad, pond-like sheets, with a current, does not make that a lake which would otherwise be a river. State v. Gilmanton, 14 N. H. 477.
-- Black's Law Dictionary