In railroad terminology, a wye is a triangular shaped arrangement of tracks with a Y-shaped switch at each corner. With a sufficiently long track leading away from each corner, a train of any length can be turned.
Important for any directional piece of railroad equipment, such as most steam locomotives, or indeed many passenger trains, especially those that have a dedicated tail end car such as an observation car.
Individual locomotives and cars can be turned on a turntable, but obviously whole trains cannot. A wye, or a loop, are the only ways of doing that.
In British railway terminology, a wye is known as a triangle, which is much more logical.