Source : Webster's Revised Unabridged Dictionary (1913)

Tarn \Tarn\, n. [OE. terne, Icel. tj["o]rn.]
   A mountain lake or pool.

         A lofty precipice in front, A silent tarn below.
                                                  --Wordsworth.

Source : WordNet®

tarn
     n : a mountain lake (especially one formed by glaciers)