In geology, the term "fold" or "folding" refers to the bending of rock layers due to stress. Most often, large folds are a result of some type of major collision.
When a rock formation experiences significant stress, the formation typically either folds or faults. Folding refers to the rock formation bending either in a syncline formation or an anticline formation. A syncline formation is concave up and resembles a "U" shape, while an anticline formation is concave down and resembles an "A" shape. The stress that forms folding in rocks is typically caused by a strong force placed on the rock formation.