grey matter

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic Brains; a person's ability to think.
• noun neuroanatomy A collection of cell bodies and (usually) dendritic connections, in contrast to white matter.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun greyish nervous tissue containing cell bodies as well as fibers; forms the cerebral cortex consisting of unmyelinated neurons