The Collaborative International Dictionary of English v.0.48:
Tarn \Tarn\, n. [OE. terne, Icel. tj["o]rn.] A mountain lake or pool. [1913 Webster] A lofty precipice in front, A silent tarn below. --Wordsworth. [1913 Webster]WordNet (r) 3.0 (2006):
tarn n 1: a mountain lake (especially one formed by glaciers)