A frame of wood, bent into the shape of the letter U, and embracing an ox's neck as a kind of collar, the upper ends passing through the bar of the yoke; also, anything so shaped, as a bend in a river. [1913 Webster]
oxbow, n.
1 a U-shaped collar of an ox-yoke.
2 a a loop formed by a horseshoe bend in a river. b a lake formed when the river cuts across the narrow end of the loop.