A nucleus in anatomy is a brain structure (plural = nuclei). It is a compact cluster of neurons. It is one of the two most common forms of nerve cell organization, the other being layered structures such as the cerebral cortex or cerebellar cortex. The same kind of structure in the peripheral nervous system is called a ganglion. Some of the traditional names for brain nuclei also use that word.
In anatomical sections, a nucleus shows up as a region of grey matter, often bordered by white matter. The vertebrate brain contains hundreds of nuclei, which vary in shape and size. A nucleus may have a complex internal structure, with several types of neurons arranged in clumps (subnuclei) or layers.