An onion skin fracture is used in geology to describe a common flaking/fracturing pattern of many rocks. It is usually used to describe fracturing caused by a reaction on the surface of a rock, which results in an expansion of the outer layer causing it to flake away from the core. This is very often the result of water either absorbing into or reacting with the rock, and is commonly seen as a result of atmospheric weathering. The term is also used to describe spheroidal weathering in saprolite.
An onion skin fracture does not indicate anything specific about the thinness of the resulting flake; while most examples tend to be flakes a few millimeters to centimeters thick, saprolite can have quite hefty chunks .