The definition is not precise, because most water bodies vary in fullness according to the season, and according to the heaviness of precipitation and other factors during any given year.
They even may be split into several separate water bodies with dry land between, either arid or covered with vegetation.
In some cases the seasonal cycle is ecologically important, whereas in other examples exceptional water levels are too rare to be taken routinely into account.
[1] Perennial standing (lentic) water is usually called a lake or a pond, but there are several local or dialect English words for such features.
Vlei is a foreign example that may or may not mean a perennial body of water, but more often than not refers to a seasonal pond.