The word occurs by itself as a noun or as a suffix forming the names of bodies of water (e.g. Rondevlei (Round vlei), Zeekoevlei (Hippopotamus vlei) or Brandvlei (Burning vlei)). It appears on occasion as a prefix, e.g. Vleifontein (Vlei Fountain). The word is difficult to translate into a single equivalent word in English, because vlei is used in a variety of senses. One approximation is "pond" but this is perhaps at the same time misleading. The word "pond" tends to conjure up images of a cool, permanent body of water under oak trees somewhere in Cambridgeshire with a pike living in it, whereas most vleis are not much like that. To some extent vlei overlaps with "lake" but once again this equivalence must be taken loosely. No Afrikaner would use the word vlei to describe Lake Superior, Lake Kariba or Loch Ness.
Vleis are never pools in rivers. (The word for that is kuil). They are not bogs or swamps either. Okefenoke would never do as a vlei. One definition of a vlei is "an isolated, open, shallow body of fresh inland water, that is not part of a watercourse and is usually reedy and typically seasonal". However, all these qualifications are to a degree arbitrary. The 'fresh' water may be quite brackish. The edges of the water may be lined with bullrushes or grass rather than reeds. The banks may even be stony or muddy.
A vlei does not cease to be a vlei when it dries up for a while. That said, a low-lying area that rarely holds water and typically has a broad, flat, bare, salty floor is a pan rather than a vlei. (The word is spelled the same in Afrikaans and English but in the former it is pronounced 'pun'). There is no absolute distinction between a vlei and a pan. Examples are Etoshapan and Verneukpan.