tarn - What does it mean?

Noun

It was possible, I reflected, that a mere different arrangement of the particulars of the scene, of the details of the picture, would be sufficient to modify, or perhaps to annihilate its capacity for sorrowful impression; and, acting upon this idea, I reined my horse to the precipitous brink of a black and lurid tarn that lay in unruffled lustre by the dwelling, and gazed down—but with a shudder even more thrilling than before—upon the remodelled and inverted images of the gray sedge, and the ghastly tree-stems, and the vacant and eye-like windows.