A tarn (or corrie loch or llyn) is a mountain lake or pool, formed in a corrie excavated by a glacier. A moraine may form a natural dam below a tarn. (A corrie may be called a cirque.)

The word is derived from the Old Norse word tjörn meaning pond as such is often found in names of both glacial and non-glacial small upland lakes in the English Lake District and Pennines.

In local dialect in Cumbria, the word is also used generally to refer to any small lake or pond, regardless of its location and origin ^*.

In Scandinavian languages a tjern or tjärn or tjørn is a small natural lake, often in a forest or with vegetation close around it or growing into the lake.

See also

Glaciology | Bodies of water

Tjern | Tjärn