Definition of Matter, gray

Matter, gray: The cortex of the brain, which contains nerve cell bodies. The gray matter is so named because it is darker than the white matter, the part of the brain that contains myelinated nerve fibers.

Source: MedTerms™ Medical Dictionary
http://www.medterms.com/script/main/art.asp?articlekey=4299
Last Editorial Review: 3/30/2012