A tarn is a defined as a mountain lake, formed in a valley that has been excavated by a glacier. The valley is usually called corrie. The word tarn has been derived from the word 'tjörn'. 'Tjörn' meaning pond, is an Old Norse word. These lakes are usually seen in the English Lake District and Pennines. This word is normally used to refer to a small pond or lake in a local dialect in Cumbria. A tjern or tjärn or tjør, in Sandivian languages, refers to a natural lake usually in a forest or surrounded by vegetation. It is also reffered to a small lake growing into a larger one. A mountain rock sometimes forms a damn below a tarn. When a valley's lip acts as a dam and stores water, it forms into a tarn.
In common use a tarn refers to a pond, usually in the mountains that often dries up during dry weather.