For some reason I think this definition means something that is relavant to this journal’s beginning(s).
The portions of the brain and spinal cord which are 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.
The definition is from The concise Biotech Dictionary.