Cleavage is a type of foliation in metamorphic rocks, characterized by the tendency of the rock to split along parallel planes. It is typically associated with low-grade metamorphism and is often found in slate and phyllite. Cleavage planes form as a result of directed pressure during deformation, making them significant in understanding the stress and strain history of a rock.
Reference: Passchier, C. W., & Trouw, R. A. J. (2005). Microtectonics. Springer.