Scientists have not agreed on the technical, formal differences between a pond and a lake, though in practice, a pond is not as deep or large as a lake. Whether a body of water is called a pond or lake usually depends on local conventions.
Scientists who study inland waters consider depth to be the defining characteristic of ponds. They propose that ponds are bodies of water where sunlight penetrates to the bottom of all areas. In contrast, lakes are so deep that sunlight does not penetrate to the bottom. On the other hand, biologists and other researchers use surface area as the defining feature. The International Ramsar Wetland Convention sets 20 acres as the largest surface area for a pond, while an organization called Pond Conservation uses five acres as the maximum size. Inland bodies of water larger than these maximums are considered lakes.
Some inland bodies of water do not fit the definitions of "pond" and "lake" convincingly. There may be some question about classifications for a body of water in which sunlight penetrates to the bottom for nine months of the year, but algae prevents sunlight from reaching the bottom in the summer months, or what to call a 100-acre body of clear, shallow water. For example, one of the most famous ponds in the United States is called Walden Pond, yet its surface area measures 61 acres and its depth is between 102 and 107 feet.