The portions of the brain and spinal cord which appear gray and composed of the main bodies of neurons (called cytons). This is in contrast to white matter, which is the portions that are white and composed of the axons and dendrites of neurons.
In the spinal cord, the gray matter is bordered by the white matter and is shaped like a butterfly when seen in cross-section.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.