a section of a trail, road, or railway line that is zig-zagged in shape, especially in order to climb up a steep hill or mountain, or one of the turns on such a route;
e.g. We hiked up the switchback to the top of the hill.
(verb)
to have complete 180° turn, especially to have multiple such turns and to therefore form a zigzag shape, describing the course of a road or the route taken by a vehicle when going up or down a steep slope;
e.g. The road began to switchback down the side of the mountain. The train started to switchback up the slope.