A pond is an area filled with water, either natural or artificial, that is smaller than a lake. It may arise naturally in floodplains as part of a river system, or be a somewhat isolated depression (such as a kettle, vernal pool, or prairie pothole). It may contain shallow water with aquatic plants and animals.
The technical distinction between a pond and a lake has not been universally standardized. Limnologists and freshwater biologists have proposed formal definitions for pond, in part to include 'bodies of water where light penetrates to the bottom of the waterbody,' 'bodies of water shallow enough for rooted water plants to grow throughout,' and 'bodies of water which lack wave action on the shoreline.' Each of these definitions has met with resistance or disapproval, as the defining characteristics are each difficult to measure or verify. Accordingly, some organizations and researchers have settled on technical definitions of pond and lake that rely on size alone.
Ponds can result from a wide range of natural processes. Any depression in the ground which collects and retains a sufficient amount of precipitation can be considered a pond, and such depressions can be formed by a variety of geological and ecological events.
A defining feature of a pond is the presence of standing water, which provides habitat for wetland plants and animals. Familiar examples might include water lilies, frogs, turtles, and herons. Often, the entire margin of the pond is fringed by wetland, and these wetlands support the aquatic food web, provide shelter for wildlife, and stabilize the shore of the pond. Some grazing animals like geese and muskrats consume the wetland plants directly as a source of food. In many other cases, however, the pond plants fall into the water and decay. Many invertebrates then feed on the decaying plants, and these invertebrates provide food for wetland species including fish, dragonflies, and herons. The open water may allow algae to grow, and these algae may support yet another food web that includes aquatic insects and minnows. A pond, therefore, may have combinations of three different food webs, one based on larger plants, one based upon decayed plants, and one based upon algae. Hence, ponds often have many different animal species using the wide array of food sources. They, therefore, provide an important source of biological diversity in landscapes.
Ponds, being small, are easily disrupted by human activity, such as hikers. Drainage of ponds is a frequent problem in agricultural areas, such as in the prairie potholes of North America. Although ponds are a useful source of water for cattle, overgrazing and wading can turn a pond into a muddy hole. Nutrient sources such as fertilized pastures, human sewage, and even lawn fertilizer can cause explosive growth of algae and the loss of rooted plants and many other aquatic species. Roads near ponds can kill large numbers of amphibians and turtles that may migrate to and from the pond as part of their annual breeding cycle. Many well-intentioned people introduce fish to ponds, being unaware that some species may eat aquatic plants, stir up sediment, and eat the young of amphibians and many other invertebrate species. The gentle slope of land into ponds also provides an expanse of habitat for wetland plants and wet meadows. The construction of retaining walls, or lawns, can severely degrade the life in a pond.
In origin, a pond is a variant form of the word pound, meaning a confining enclosure. As straying cattle are enclosed in a pound so water is enclosed in a pond. In earlier times, ponds were artificial and utilitarian, as stew ponds, mill ponds and so on. The significance of this feature seems, in some cases, to have been lost when the word was carried abroad with emigrants. In the United States, natural pools are often called ponds. Ponds for a specific purpose keep the adjective, such as "stock pond", used for watering livestock.
Ponds are used for the provision of fish and other wildlife including waterfowl, which is a source of food for humans. Pollutants entering ponds are often substantially mitigated by the natural sedimentation and biological activities within the water body. Ponds are also a major contributor to local ecosystem richness and diversity for both plants and animals.
In medieval times in Europe, it was typical for many monastery and castles (small, partly self-sufficient communities) to have fish ponds. These are still common in Europe and in East Asia (notably Japan), where koi may be kept.