Etymology, meaning, and usage of "lake"
The Caspian Sea
is either the world's largest lake or a full-fledged sea.
The word lake comes from Middle English lake ("lake, pond, waterway"), from Old English lacu ("pond, pool, stream"), from Proto-Germanic *lakō ("pond, ditch, slow moving stream"), from the Proto-Indo-European root *leǵ- ("to leak, drain"). Cognates include Dutch laak ("lake, pond, ditch"), Middle Low German lāke ("water pooled in a riverbed, puddle") as in: de:Wolfslake, de:Butterlake, German Lache ("pool, puddle"), and Icelandic lækur ("slow flowing stream"). Also related are the English words leak and leach.
There is considerable uncertainty about defining the difference between lakes and ponds, and no current internationally accepted definition of either term across scientific disciplines or political boundaries exists. For example, limnologists have defined lakes as water bodies which are simply a larger version of a pond, which can have wave action on the shoreline or where wind-induced turbulence plays a major role in mixing the water column. None of these definitions completely excludes ponds and all are difficult to measure. For this reason, simple size-based definitions are increasingly used to separate ponds and lakes. One definition of lake is a body of water of 2 hectares (5 acres) or more in area;:331 however, others have defined lakes as waterbodies of 5 hectares (12 acres) and above, or 8 hectares (20 acres) and above  (see also the definition of "pond"). Charles Elton, one of the founders of ecology, regarded lakes as waterbodies of 40 hectares (99 acres) or more. The term lake is also used to describe a feature such as Lake Eyre, which is a dry basin most of the time but may become filled under seasonal conditions of heavy rainfall. In common usage, many lakes bear names ending with the word pond, and a lesser number of names ending with lake are in quasi-technical fact, ponds. One textbook illustrates this point with the following: "In Newfoundland, for example, almost every lake is called a pond, whereas in Wisconsin, almost every pond is called a lake."
One hydrology book proposes to define the term "lake" as a body of water with the following five characteristics:
- it partially or totally fills one or several basins connected by straits
- has essentially the same water level in all parts (except for relatively short-lived variations caused by wind, varying ice cover, large inflows, etc.)
- it does not have regular intrusion of seawater
- a considerable portion of the sediment suspended in the water is captured by the basins (for this to happen they need to have a sufficiently small inflow-to-volume ratio)
- the area measured at the mean water level exceeds an arbitrarily chosen threshold (for instance, one hectare)
With the exception of the seawater intrusion criterion, the others have been accepted or elaborated upon by other hydrology publications.