The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It covers the cerebrum and cerebellum, and is divided into left and right hemispheres. The cerebral cortex plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It consists of up to six horizontal layers, each with a different composition in terms of neurons and connectivity. The human cerebral cortex is 2–4 mm (0.08–0.16 inches) thick.
In preserved brains, it has a grey colour, hence the name "grey matter". In contrast to gray matter that is formed from neurons and their unmyelinated fibers, the white matter below them is formed predominantly by myelinated axons interconnecting neurons in different regions of the cerebral cortex with each other and neurons in other parts of the central nervous system.
The surface of the cerebral cortex is folded in large mammals, such that more than two-thirds of it in the human brain is buried in the grooves, called "sulci". The phylogenetically most recent part of the cerebral cortex, the neocortex (also called isocortex), is differentiated into six horizontal layers; the more ancient part of the cerebral cortex, the hippocampus (also called archicortex), has at most three cellular layers, and is divided into subfields. Neurons in various layers connect vertically to form small microcircuits, called columns. Different neocortical architectonic fields are distinguished by variations in the thickness of these layers, their predominant cell type and other factors such as neurochemical markers.